there are some old desktop boards (like Asus D810) which have two sockets but at a time only one socket will be functional. so for the desktop PC is clearly NO at all.in short the answer is no
if its just a desktop, HP, Acer, asus etc then the chances of it having the second socket is pretty much non-existant
Why not just use 1 processor that has multiple cores? If you need speed go for a Phenom II X6 (6-core), Phenom II X4 (4-core), Phenom II X2 (2-core), core i3, core i5 or core i7.
Besides that, I can't imagine what anyone would do with or even need a core i7 990 for. So, for a desktop, I think, that is the king of the hill. If you need more speed, then, you're doing something a server would probably be better at. However, I don't think the OP needs that much power. Maybe he had a couple of CPUs lying around and just wanted to put them to use.