My dd did this at 12U. I combined my best 5 from the 10U team I coached with the best 5 from our fiercest 10U rival. I knew going in that my role would be as assistant coach. I had to cut one of my top 5 because I knew the parents would not mesh with the other team. We had two really good years before the team split up (amicably) at 14U. It takes mature coaches, parents and players for this to work. If there are egos and/or drama involved before the merger, I would be concerned.