Estoy tratando de fusionar dos modelos secuenciales en Keras 2.0, usando la siguiente línea:

merged_model.add(Merge([model1, model2], mode='concat'))

Esto todavía funciona bien, pero da una advertencia:

"The `Merge` layer is deprecated and will be removed after 08/2017. Use
instead layers from `keras.layers.merge`, e.g. `add`, `concatenate`, etc." 

Sin embargo, estudiar la documentación de Keras e intentar agregar, Add (), no ha resultado en algo que funcione. He leído varias publicaciones de personas con el mismo problema, pero no encontré ninguna solución que funcione en mi caso a continuación. ¿Alguna sugerencia?

model = Sequential()
model1 = Sequential()
model1.add(Dense(300, input_dim=40, activation='relu', name='layer_1'))
model2 = Sequential()
model2.add(Dense(300, input_dim=40, activation='relu', name='layer_2'))
merged_model = Sequential()

merged_model.add(Merge([model1, model2], mode='concat'))

merged_model.add(Dense(1, activation='softmax', name='output_layer'))
merged_model.compile(loss='binary_crossentropy', optimizer='adam', 
metrics=['accuracy'])

checkpoint = ModelCheckpoint('weights.h5', monitor='val_acc',
save_best_only=True, verbose=2)
early_stopping = EarlyStopping(monitor="val_loss", patience=5)

merged_model.fit([x1, x2], y=y, batch_size=384, epochs=200,
             verbose=1, validation_split=0.1, shuffle=True, 
callbacks=[early_stopping, checkpoint])

EDITAR: cuando lo intenté (como lo sugiere Kent Sommer a continuación):

from keras.layers.merge import concatenate
merged_model.add(concatenate([model1, model2]))

Este fue el mensaje de error:

Traceback (most recent call last):
  File "/anaconda/lib/python3.6/site- packages/keras/engine/topology.py", line 425, 
in assert_input_compatibility
    K.is_keras_tensor(x)
  File "/anaconda/lib/python3.6/site-
packages/keras/backend/tensorflow_backend.py", line 403, in     is_keras_tensor
    raise ValueError('Unexpectedly found an instance of type `' +
 str(type(x)) + '`. '
ValueError: Unexpectedly found an instance of type 
`<class'keras.models.Sequential'>`. Expected a symbolic tensor instance.

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "quoradeeptest_simple1.py", line 78, in <module>
    merged_model.add(concatenate([model1, model2]))
  File "/anaconda/lib/python3.6/site-packages/keras/layers/merge.py",
 line 600, in concatenate return Concatenate(axis=axis, **kwargs)(inputs)
  File "/anaconda/lib/python3.6/site-   packages/keras/engine/topology.py", 
line 558, in __call__self.assert_input_compatibility(inputs)
  File "/anaconda/lib/python3.6/site-packages/keras/engine/topology.py", line 431, 
 in assert_input_compatibility str(inputs) + '.All inputs to the layer '
ValueError: Layer concatenate_1 was called with an input that isn't a
symbolic tensor. Received type: <class 'keras.models.Sequential'>. 
Full input: [<keras.models.Sequential object at 0x140fa7ba8>,
<keras.models.Sequential object at 0x140fabdd8>]. All inputs to the
layer should be tensors.