If your PSU is capable of delivering at least 850W of power to the entire system with the two in SLI, then yes they can go in. My other concern is that you won't get anything out of SLI'ing and I'm worried about the motherboard you're working with, if it has support for SLI.
Links to both cards? IMHO, you're only going to dump more heat, as opposed to buying the single most powerful card and dumping less heat in the process.