Are you using the same driver for both boards? I have never tried two ATI cards but that is what is generally suggested for two nvidia cards.
If you just want 3 monitors I would suggest a single uncomplicated $120 nVidia card like the GTX 650 like this
As far as I know about Windows PC's, they cannot operate with two different video cards inside, they will not be able to run two different GPU drivers at the same time. So I would say the crashing got nothing to do with PPRO, but just with the fact that probably you have an impossible hardware setup?
As already stated by Bill, 1 week ago I just bought a simple Nvidia card for $160 (I was on an ATI triple screen card before, with three screens connected which actually worked because it is just one single card designed for / with a single driver for a tripple screen setup). And I am just amazed at the performance gain I got just from having the Cuda support now, for realtime playback of the video edit effects, and in final output rendering times. I bought the Asus GTX660 DCII OC PH 2GB. And connected the same tripple screens back to it, the manual states it can actually run even 4 screens simultaneously.
Just study Harm Millards articles on the tweakers page where he explains "Balanced Systems". I think the quality of his research and presenting of this information is about the best you can find for free on the web, most questions will be answered there for many who come to the hardware forum here.
Thanks for the reply. I'm trying not to spend any more money on my ancient computer, but I'll certainly consider one of those GPUs when I rebuild.