如何用Keras实现深双向LSTM？

I am trying to implement a LSTM based speech recognizer. So far I could set up bidirectional LSTM (i think it is working as a bidirectional LSTM) by following the example in Merge layer. Now I want to try it with another bidirectional LSTM layer, which make it a deep bidirectional LSTM. But I am unable to figure out how to connect the output of the previously merged two layers into a second set of LSTM layers. I don't know whether it is possible with Keras. Hope someone can help me with this.I am trying to implement a LSTM based speech re