Problem solved: I had to switch the cards positions. The slower card needed to be in pci-e #1. It had to be some timing issue.
Switching cards is the #1 thing to try when experiencing SLI problems... Weird stuff though. I suppose it's somehow linked to each card's BIOS acting a bit differently than the reference design.
To answer your last question, I favor F.E.A.R. and 3DMark06's Canyon Flight benchmarks to check SLI functionality. Among your options, I'd pick Crysis since it eats as much GPU power as you can throw it and is an official TH2G-compatible title.