Sequence forecasting in keras not possible for variable-length sequence forecasts
No, it is not possible with Vanilla LSTM. It's not a Kera problem, but the fundamental structure of Vanilla LSTM. 

I think you have a bit of misunderstanding of how LSTM works. You are assuming given $x_0$...$ x_{19}$ the LSTM is predicting $x_{20}$...$x_{39}$. This is NOT the case. In LSTM if your input sequence is $x_0$...$ x_{19}$ then your output sequence is $x_1$...$ x_{20}$. 

To achieve what you want, you can use something like Seq2Seq model.