I've just come up with a solution to this... Create a copy of your 'Vu+ Duo' device. Learn each button in both 'devices' one after another, jumping from one to the other in between.

eg. 'Learn' UP_BUTTON on "VU a", then go to your "VU b" and learn it on there (making sure not to press anything else in between).
Then in 'VU a', add "Jump to VU b" and vice versa.

The screen change isn't even visible. Yes it's a bit of a pain to set up, but works well once done. :)