I currently use a cheap little USB switch to switch my mouse and keyboard between a GPU powered HVM and normal Qubes interface. My setup includes a secondary display that will always be Qubes and a primary that will switch between Qubes and HVM as needed. And I was thinking it would be really nifty to be able to simply go from one screen to another without having to wait for a switch.
Synergy and Barrier claim to do just this between two different computers. And if that’s the case, I imagine it’s possible to do the same between two different virtual machines.
From what I can tell, both machines would need to be able to access the LAN, so that would seem to preclude putting it on Dom0 (not that I’d necessarily want to install nonstandard stuff there anyway), but I think the new Qubes paradigm includes a dedicated USB qube (I either installed before that was an option or just didn’t choose to create one), so maybe there’s a clever way to go about this without entirely defeating the whole ‘security by isolation’ thing by the end of the day.