Pairing up maroon's is risky business...I'm not sure a 20Long is really big enough for maroons, but I believe I've seen it done successfully before.
I think your best bet would be to get two that are already paired up, however, if that's not an option, getting one that is noticeably larger than the other will increase your chances of a successful pairing. I'm not sure it really matters whether they're from the same tank at the LFS or not.