no it won't, on any setup the clocks can be different on each card. Memory will reduce, say one is 1GB, the ohter is 512mb, then you will have 2 cards iwth 512mb, rather than 1 with 1024mb, one with 512mb...if that makes sense
to OP though:
Stick with a single card, and stick with your 5850. There really is no point in upgrading at all. A 5970 would give better performance, but I personally wouldn't be able to justify upgrading a card that can play any game on full, to a card that can play any game on full, and pay a small fortune to have the pleasure of having no difference, except for maybe an extra few fps in games, but with fps being high anyway, it isn't worth it.
Wait for fermi to come out and see how they are priced, or, see if ATi cards drop in price, then it may be worth getting a 5970, but at the moment, no, I wouldn't recomend doing it