I use a K2200 and a 970 with good results in Pr.
Not so good in Ae. One surprise I got with both the 970 and K2200 Quadro was you can't do ray-traced 3D, which means, essentially no native 3D in Ae. You have to use the (included) Maxon software outside the application. Or CPU rendering which, even on an 8 core i7 is too slow to be useful. No extrusion is possible without using the short list of expensive, and per the hardware forum, poorly performing for their cost, "approved" cards.
You could run 2 cards in SLI mode if they are the same model. I don't think your solution would work.
Well yes, I read that only those "supported" cards are actually able to perform ray-traced 3D.
But since I will have to edit .R3D files soon, I need to stack up my pc.
Any experience in this direction?
The Hardware Forum has links to the Tweakers page, where I learned it's most important to build a balanced system. If you get bottlenecks in one area lots of horsepower in another won't help. I'm not the expert, but following the directions there I built a pretty quick system. SSD C and D drives, 8 striped 7200 rpm RAID drives, 64GB RAM, two graphics cards, an 8 core I7. I get the impression the CPU is more important than the GPU. There is also a link on the Tweakers page to a set of files you can download to test your system and benchmark it against others. It will show where you are strong and weak.
AE does not use a single or even two video cards. Everything AE is done by the CPU, so having two video cards instead of one, only doubles idle cards.
PR can use two (or more) video cards, but SLI is not required, nor is it desirable. The benefit of two high-end video cards is doubtful in most cases, since rendering for export / encoding is done by the CPU, not the GPU and as Barton mentioned the rest of the system must be up to the task to remain balanced.