SLI is essentially a gaming technology, and you need a software profile optimized for an application to make it work really well. There are none for Poser nor Vue. Plus, in the case of many Poser figures loaded at the same time the burden is also very much on the CPU. I tested many 3D content creation apps on SLI systems, it was always extremely disappointing. 

But when you play on a high res monitor (1920 x 1200 and above) with heavy games like Oblivion or FEAR, now that's another story of course, SLI (or Crossfire) is great.