We did it last year. My DD played majors in LL (11-12 yr olds) and played mainly 14U travel. The only time it was a real big problem was during LL all-stars, because LL has a stupid rule that you can't play in a different league during all stars (from first game til your team is done). Her team made it to the state semi's, so it ended up being several weeks, which kept her from playing with a 12U team (that she subbed for earlier in the year) in the ASA state tournament.
She wants to play LL again this year, because of her friends. She is a 12 year old, but from my understanding of the rule changes listed on the LL website, 12 year olds have the option of playing majors or juniors, which is now 12, 13, and 14 year olds. I'm gonna try to talk her into playing Juniors, because she should be playing with the older girls anyway, and because they don't have the rule about other leagues for the junior division.