Hmm...
I still argue that you don't need threaded rendering on the GUI level.
What each GPU is doing in parallel, is still out of Qt's 'realm' - since you are directing in opengl commands to the respective GPU (i guess).
Since you are rendering on multiple screens, you are rendering multiple widgets, and I don't know if this is at all possible to do (in sync) in parallel with out special hardware. (I am not sure the underlying window manager can do parallel window rendering - and maybe you should check that first).

But I don't know if this kind of question and contemplation is out of this forums scope (I mean, it is a valid Qt question, but I doubt any one not very well versed in the details of your system can really answer).
The answer is probably a design issue, rather than implementation detail.
Once you have the correct design (taking Qt abilities in consideration) one can think about how to go and implement it - where threaded rendering may or may not come to be a question...

But, maybe all I have said here is totally wrong, since I know very little about your system.