Assuming you are in the amp, first touch the icon on the top left that looks like 5 grey bars stacked in a square.
2 touch the plus sign at the bottom left
3 add Guitar amp
4 Record as normal.(It may play the other part while you are recording but this can be muted)
Thanks Supotco! I did a real quick test, without plugging but just using the external mic and speakers and it seemed to do what I want. Nice! I'll be trying it with a full setup a bit later this afternoon and let you know if I was successful.