model = Sequential()
model.add(InputLayer((1, H, W)))

model.add(Convolution2D(64, 5, 5, border_mode = 'same', activation = 'relu'))
model.add(Convolution2D(64, 5, 5, border_mode = 'same', activation = 'relu'))
last = Convolution2D(64, 5, 5, border_mode = 'same', activation = 'relu')
model.add(last)

a_model = Sequential()
a_model.add(last)
a_model.add(Convolution2D(64, 5, 5, border_mode = 'same', activation = 'relu'))
a_model.add(Convolution2D(64, 5, 5, border_mode = 'same', activation = 'relu'))
a_model.add(Convolution2D(1, 3, 3, border_mode = 'same', activation = 'sigmoid'))

b_model = Sequential()
b_model.add(last)
b_model.add(Convolution2D(64, 5, 5, border_mode = 'same', activation = 'relu'))
b_model.add(Convolution2D(64, 5, 5, border_mode = 'same', activation = 'relu'))
b_model.add(Convolution2D(1, 3, 3, border_mode = 'same', activation = 'sigmoid'))

model.add(Merge((a_model, b_model), mode = 'concat'))

Using Theano backend.
Using gpu device 0: GeForce GTX TITAN (CNMeM is disabled, cuDNN 5004)
Traceback (most recent call last):
  File "/home/chase/workspace/Colorizer/colorizer2.py", line 79, in <module>
    model.add(Merge((a_model, b_model), mode = 'concat'))
  File "/usr/local/lib/python3.5/dist-packages/keras/engine/topology.py", line 1118, in __init__
    self.add_inbound_node(layers, node_indices, tensor_indices)
  File "/usr/local/lib/python3.5/dist-packages/keras/engine/topology.py", line 524, in add_inbound_node
    assert len(node_indices) == len(inbound_layers)
AssertionError