I wonder if you just have a cross-talk problem?   Are you by chance putting the same outfit or item on both figures?  If somehow the two items end up infinitely referencing each other, Poser would run out of memory even if you have 1024GB of ram (eventually anyway :-) )

I think it's clear no one here thinks you need an 8GB Vista x64 system just to render 2 V4.2 figures (I have XP-Pro w 4GB, Quad-core and I often do 2 V4.2 and up to 5 A3's in one scene - no problem, other than the PZ3/PMD's get 100MB+)

However, I have had bad objects stop renders - often with no message at all.  The Render just runs 5 or 10 minutes, then shows me a gray image, happy as can be. 

Things which have stopped me in the last month:

1. I placed a texture on part of spunky's space suit for A3 ... no more renders ever completed, no message; clear the texture, still no joy. Delete the suit, create a new one and stick to solid colors, no problem.  What was wrong? Don't know.
2. I now have some silly halloween candy bag, but with three in one scene, applying a MAT always effects the first bag only, plus same thing. Renders never complete, but I get some cryptic message about (cannot find texture "") with no name.  Delete all three bags, rendering works, add three more and leave coloring to postwork, all is fine.

As for the USB limit, I'd heard that but I think it depends on the software/hardware versions because I have several 20+GB files on my external USB drive (they are encrypted TrueCrypt files).  They work fine.