Hello, apologies in advance but I am an engineer not a mathematician(!)

I’m working on an acoustics problem of a cube with one face radiating.

I based it on this tutorial: https://nbviewer.org/github/mscroggs/bempp-acoustic-tutorials/blob/main/tutorials/5_loudspeaker.ipynb

It works quite nicely, but now I would like to try making the opposite face radiate with an amplitude and phase difference to look at the directivity patterns and generally play around with it.

I have my tagged mesh set up with different IDs for the different faces.

How do I set up the two velocity fields in Bempp with different amplitude and phase?

How do I set up the solver with the multiple spaces?

Thanks!