Error: keras merge LSTM layers in sum mode
The error says what's the problem: the method expects a Tensors, but you are giving a Sequential model object.

Use functional model (from keras.models import Model), not Sequential.

Then, merge the models with:

merged_models = Model(inputs=[first_model_input, second_model_input], outputs=[first_model_output, second_model_output])


or whatever your input looks like.