Recently, I got another HD 5870 to crossfire with the original one in my machine. They are made by different manufacturers but everything else is the same. I tested each card individually and they both worked fine. Unfortunately, if I have both in the system at the same time, it won't post. I know the power supply and video cards are not the issue but i can't real
This is very, very likely to be a power supply related issue in my experience.
Two high end video cards draw a LOT of power:
Going from one 5870 to two means +170w under load, and +26w at idle.