I got 64GB... but dual renders would be crazy to even attempt without SLI engaged and at least two GPU cards.
My reasons are more mundane: I use one to pound out scripts (right now the 2nd instance is grinding through a G8F -> G3F batch pose converter script with like 8,000 pose files), and the other one I can use to tinker with and run renders on.
Can also alternate renders between them if I wanted (render on one, tinker on the other, then switch.) I wouldn't run more than two instances, though, for the reason you mentioned.
Also, I don't use Iray preview, so there's no real load on the GPU unless I'm rendering on both (which could be done I guess, but only if you had really, really light scenes and at least 8GB on the GPU card.)