Well I tried this out and got shit for results because of 2 reasons:
1. Different resolutions for the monitors. I have a 27" 2560x1440 as my main monitor and 2 23" 1920x1080 on either side of the main one. And a further complication of:
2. I use ISBoxer (Innerspace) to have 5 smaller windows for each toon displayed on my right side monitor and then I can switch characters (switch seamlessly) and still see what the other toons are doing or NOT doing, lol. Basically I have all 6 toons visible at the same time on 2 different monitors.
The difference in resolutions of the monitors and switching back and forth between toons at those different resolutions is what caused major headaches with Viewports enabled. I really like the ability to setup a Windowed profile for each toon. One of the main reasons I use ISB to begin with. So I gave up on trying to get Viewports to work, because it was causing all kinds of crazy problems that I didn't want to deal with. I could get the main monitor to display Viewports properly but the second I switched between toons hosed the whole thing. 2 screens would go blank (black) and nothing I did would bring them back, short of logging out all toons and relogging back in.
That's my 2 Redcents worth, oh and I make no claims to being even remotely proficient at this stuff, just did enough research to at least understand my problem.
Vrak