I've not experienced this myself, thankfully, but have you tried the WoW support forums (*crosses self*):

Error #134[
Solution for one person (although I see you've already tried disabling all your mods).

What puzzles me most about your reported problem is that you say both windows crash at the same time, even though you're running them from entirely separate folders? Are you positive that you're running them from different folders (i.e. you didn't accidentally use the same shortcut twice)?

Have you symlinked / junctioned the data folder, or do you have two full, entirely separate, installations?

Are they both trying to read the SAME address, or is the address different for each?

Do they really crash at exactly the same time?

Have you tried the repair tool? Don't let it reset your mods, etc, just do a file repair and see what it says. Run it on both installations of WoW.

If they're totally separate and the repair tool doesn't find any problems, it would suggest something that's mapped into the memory of both processes, and quite honestly I've no idea how to go about diagnosing that!

It could still be a hardware-related problem - if you're so inclined then you could download Memtest and run that to exercise your RAM for example - but the fact that it affects both processes simultaneously suggests that's less likely, although as I write that I doubt myself. A memtest is never a bad thing in any case.

If all else fails, it's worth a post on the WoW forums - the Blues are quite responsive to that sort of thing (remember to post the full error log from your Errors folder over there).