The answers will vary. I have twins, and, before they were born I read that when multiples sleep together they have stronger vital signs and sleep better. But, other things I've read said do not have siblings sleep together because of the risk of SIDS (although, I think this last part was aimed more for older siblings). We kept our boys in the same crib until they were seven weeks old. The general consensus I've found with my multiples friends is that you can keep them together until they start to move around in the crib or seem cramped.
A word of advice! In my multiples group, we've discussed the ages that twins were separated into their own cribs. Moms who went past the four month mark really wish they would have separated earlier. Their babies seemed to have a much more difficult transition than those of us who did it sooner.