Although I agree that it depends on the individuals...I can see the flip side (in this case the negative side).
My nani and my dadi were sisters, and my nana-abbu and dada-abbu were brothers. Unfortunately due to my dadi's very self-destructive personality there was always clashes in the household. However, they lived in a joint family system (we are talking about 1950's Pakistan). If two sisters got married to two brothers and they lived fairly separate lives its all good.
Yeah, I think that even if two sisters are married to two brothers, everyone needs some space in order to make things work. I can see how being in too close proximity can cause problems, but that's just as likely even in any situation.