diff --git a/pytorch_lightning/trainer/data_loading.py b/pytorch_lightning/trainer/data_loading.py
index 5f2cb3a894..449102fba2 100644
--- a/pytorch_lightning/trainer/data_loading.py
+++ b/pytorch_lightning/trainer/data_loading.py
@@ -29,6 +29,7 @@ from pytorch_lightning.utilities.debugging import InternalDebugger
 from pytorch_lightning.utilities.model_utils import is_overridden
 from pytorch_lightning.utilities.xla_device_utils import XLADeviceUtils
 from copy import deepcopy
+from typing import Iterable
 
 TPU_AVAILABLE = XLADeviceUtils.tpu_device_exists()
 try:
@@ -336,7 +337,19 @@ class TrainerDataLoadingMixin(ABC):
             The dataloader
         """
         dataloader = dataloader_fx()
+        dataloader = self._flatten_dl_only(dataloader)
 
         if self.accelerator_backend is not None:
             self.accelerator_backend.barrier('get_dataloaders')
         return dataloader
+
+    def _flatten_dl_only(self, dataloaders):
+        # handles user error when they return:
+        # return dl1, dl2  vs  return (dl1, dl2)
+        if isinstance(dataloaders, tuple):
+            all_dls = [isinstance(x, Iterable) for x in dataloaders]
+            all_dls = all(all_dls)
+            if all_dls:
+                dataloaders = list(dataloaders)
+
+        return dataloaders
diff --git a/pytorch_lightning/trainer/evaluation_loop.py b/pytorch_lightning/trainer/evaluation_loop.py
index 8d65561ad0..17630953a8 100644
--- a/pytorch_lightning/trainer/evaluation_loop.py
+++ b/pytorch_lightning/trainer/evaluation_loop.py
@@ -144,14 +144,22 @@ class EvaluationLoop(object):
         # make dataloader_idx arg in validation_step optional
         args = [batch, batch_idx]
 
-        multiple_val_loaders = (not test_mode and len(self.trainer.val_dataloaders) > 1)
-        multiple_test_loaders = (test_mode and len(self.trainer.test_dataloaders) > 1)
+        multiple_val_loaders = (not test_mode and self._get_num_dataloaders(self.trainer.val_dataloaders) > 1)
+        multiple_test_loaders = (test_mode and self._get_num_dataloaders(self.trainer.test_dataloaders) > 1)
 
         if multiple_test_loaders or multiple_val_loaders:
             args.append(dataloader_idx)
 
         return args
 
+    def _get_num_dataloaders(self, dataloaders):
+        # case where user does:
+        # return dl1, dl2
+        length = len(dataloaders)
+        if len(dataloaders) > 0 and isinstance(dataloaders[0], (list, tuple)):
+            length = len(dataloaders[0])
+        return length
+
     def evaluation_step(self, test_mode, batch, batch_idx, dataloader_idx):
         # configure args
         args = self.build_args(test_mode, batch, batch_idx, dataloader_idx)
diff --git a/tests/trainer/dynamic_args/__init__.py b/tests/trainer/dynamic_args/__init__.py
new file mode 100644
index 0000000000..e69de29bb2
diff --git a/tests/trainer/dynamic_args/test_multiple_eval_dataloaders.py b/tests/trainer/dynamic_args/test_multiple_eval_dataloaders.py
new file mode 100644
index 0000000000..385c18a50b
--- /dev/null
+++ b/tests/trainer/dynamic_args/test_multiple_eval_dataloaders.py
@@ -0,0 +1,154 @@
+from pytorch_lightning import Trainer
+from tests.base.boring_model import BoringModel
+import torch
+from torch.utils.data import Dataset
+
+
+class RandomDatasetA(Dataset):
+    def __init__(self, size, length):
+        self.len = length
+        self.data = torch.randn(length, size)
+
+    def __getitem__(self, index):
+        return torch.zeros(1)
+
+    def __len__(self):
+        return self.len
+
+
+class RandomDatasetB(Dataset):
+    def __init__(self, size, length):
+        self.len = length
+        self.data = torch.randn(length, size)
+
+    def __getitem__(self, index):
+        return torch.ones(1)
+
+    def __len__(self):
+        return self.len
+
+
+def test_multiple_eval_dataloaders_tuple(tmpdir):
+    class TestModel(BoringModel):
+
+        def validation_step(self, batch, batch_idx, dataloader_idx):
+            if dataloader_idx == 0:
+                assert batch.sum() == 0
+            elif dataloader_idx == 1:
+                assert batch.sum() == 11
+            else:
+                raise Exception('should only have two dataloaders')
+
+        def training_epoch_end(self, outputs) -> None:
+            # outputs should be an array with an entry per optimizer
+            assert len(outputs) == 2
+
+        def val_dataloader(self):
+            dl1 = torch.utils.data.DataLoader(RandomDatasetA(32, 64), batch_size=11)
+            dl2 = torch.utils.data.DataLoader(RandomDatasetB(32, 64), batch_size=11)
+            return [dl1, dl2]
+
+    model = TestModel()
+    model.validation_epoch_end = None
+
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        limit_train_batches=2,
+        limit_val_batches=2,
+        max_epochs=1,
+        row_log_interval=1,
+        weights_summary=None,
+    )
+
+    trainer.fit(model)
+
+
+def test_multiple_eval_dataloaders_list(tmpdir):
+    class TestModel(BoringModel):
+
+        def validation_step(self, batch, batch_idx, dataloader_idx):
+            if dataloader_idx == 0:
+                assert batch.sum() == 0
+            elif dataloader_idx == 1:
+                assert batch.sum() == 11
+            else:
+                raise Exception('should only have two dataloaders')
+
+        def val_dataloader(self):
+            dl1 = torch.utils.data.DataLoader(RandomDatasetA(32, 64), batch_size=11)
+            dl2 = torch.utils.data.DataLoader(RandomDatasetB(32, 64), batch_size=11)
+            return dl1, dl2
+
+    model = TestModel()
+    model.validation_epoch_end = None
+
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        limit_train_batches=2,
+        limit_val_batches=2,
+        max_epochs=1,
+        row_log_interval=1,
+        weights_summary=None,
+    )
+
+    trainer.fit(model)
+
+
+def test_multiple_optimizers_multiple_dataloaders(tmpdir):
+    """
+    Tests that only training_step can be used
+    """
+    class TestModel(BoringModel):
+        def on_train_epoch_start(self) -> None:
+            self.opt_0_seen = False
+            self.opt_1_seen = False
+
+        def training_step(self, batch, batch_idx, optimizer_idx):
+            if optimizer_idx == 0:
+                self.opt_0_seen = True
+            elif optimizer_idx == 1:
+                self.opt_1_seen = True
+            else:
+                raise Exception('should only have two optimizers')
+
+            self.training_step_called = True
+            loss = self.step(batch[0])
+            return loss
+
+        def training_epoch_end(self, outputs) -> None:
+            # outputs should be an array with an entry per optimizer
+            assert len(outputs) == 2
+
+        def validation_step(self, batch, batch_idx, dataloader_idx):
+            if dataloader_idx == 0:
+                assert batch.sum() == 0
+            elif dataloader_idx == 1:
+                assert batch.sum() == 11
+            else:
+                raise Exception('should only have two dataloaders')
+
+        def val_dataloader(self):
+            dl1 = torch.utils.data.DataLoader(RandomDatasetA(32, 64), batch_size=11)
+            dl2 = torch.utils.data.DataLoader(RandomDatasetB(32, 64), batch_size=11)
+            return dl1, dl2
+
+        def configure_optimizers(self):
+            optimizer = torch.optim.SGD(self.layer.parameters(), lr=0.1)
+            optimizer_2 = torch.optim.SGD(self.layer.parameters(), lr=0.1)
+            return optimizer, optimizer_2
+
+    model = TestModel()
+    model.validation_epoch_end = None
+
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        limit_train_batches=2,
+        limit_val_batches=2,
+        max_epochs=1,
+        row_log_interval=1,
+        weights_summary=None,
+    )
+
+    trainer.fit(model)
+    assert model.opt_0_seen
+    assert model.opt_1_seen
diff --git a/tests/trainer/dynamic_args/test_multiple_optimizers.py b/tests/trainer/dynamic_args/test_multiple_optimizers.py
new file mode 100644
index 0000000000..eb95f2a43a
--- /dev/null
+++ b/tests/trainer/dynamic_args/test_multiple_optimizers.py
@@ -0,0 +1,50 @@
+from pytorch_lightning import Trainer
+from tests.base.boring_model import BoringModel
+import torch
+
+
+def test_multiple_optimizers(tmpdir):
+    """
+    Tests that only training_step can be used
+    """
+    class TestModel(BoringModel):
+        def on_train_epoch_start(self) -> None:
+            self.opt_0_seen = False
+            self.opt_1_seen = False
+
+        def training_step(self, batch, batch_idx, optimizer_idx):
+            if optimizer_idx == 0:
+                self.opt_0_seen = True
+            elif optimizer_idx == 1:
+                self.opt_1_seen = True
+            else:
+                raise Exception('should only have two optimizers')
+
+            self.training_step_called = True
+            loss = self.step(batch[0])
+            return loss
+
+        def training_epoch_end(self, outputs) -> None:
+            # outputs should be an array with an entry per optimizer
+            assert len(outputs) == 2
+
+        def configure_optimizers(self):
+            optimizer = torch.optim.SGD(self.layer.parameters(), lr=0.1)
+            optimizer_2 = torch.optim.SGD(self.layer.parameters(), lr=0.1)
+            return optimizer, optimizer_2
+
+    model = TestModel()
+    model.val_dataloader = None
+
+    trainer = Trainer(
+        default_root_dir=tmpdir,
+        limit_train_batches=2,
+        limit_val_batches=2,
+        max_epochs=1,
+        row_log_interval=1,
+        weights_summary=None,
+    )
+
+    trainer.fit(model)
+    assert model.opt_0_seen
+    assert model.opt_1_seen