A common practice when wanting a pair of fish is to buy several, wait for a pair to form, and then either sell the others or take them back to the lfs. That being said, out of the 6 rams I have had (2 at a time) only once did they not pair off and spawn. If you aren't looking to raise fry, though, that isn't necessarily a bad thing. My one pair that didn't spawn shared the tank peacefully, and I wasn't wanting to breed them, so I didn't really care if they paired off or not.
As for 2 pairs in a 29, that might be pushing it. I think I remember reading that you want around 20 gallons a pair, to keep them from fighting each other. Different people have different results, though. If you put them in a very heavily planted tank that breaks up line of sight, or if you get lucky and get laid back fish, then it could work.