I don’t know if there is a standard method to do this in the literature, but I can think of 2 approaches to the problem.
Supposing you are interested only in the last hidden state of both LSTMs you can:

Concatenate LSTM’s hidden states in one hidden state and feed it to your MLP.
Sum or average the LSTM’s hidden states: this is done for example when you use a bidirectional LSTM to merge the forward and the backward features. Feed the result to your MLP.