I have B/G twins, so it may be a bit different, but I potty trained them seperately. My girl was ready before my boy, so she went 1st, but I always had a 2nd potty for him, just in case he wanted to try. It was a huge motivation for him to see her getting treats (M&Ms) when she went on the potty, and even more when she got to carry the potty to the toilet and dump it out and flush (I found the dump/flush a huge motivator for all my kids). BTW, it took him only 2 days to train after watching her do it. I would start with the twin that seems most interested or talks about the potty the most. Good luck...you'll be free of diaper changes soon :)