HELP!!!

So, about three weeks after we brought Kieran home, Braxton started waking up at night. At least two times a night, and independantly of his brother. So we know that Kieran isn't waking Braxton up with crying (he doesn't cry too much at night anyway), but what we don't know is why Braxton all of a sudden wakes up 2-4 times a night. Does anybody know? Or have any ideas as to how to stop this???? Braxton was a great sleeper until Kieran came home. One baby waking me up several times a night is MORE than enough. Please help a mommy in distress!