We had a teammate of our son go to the same school, but in our case we were very happy about it.
I have heard of others in this situation. But if you're already committed, typically the coaches will mention their interest in the other player and, if they don't ask you point blank for your take on him, they'll at least await your reaction.
You have to tread very, very lightly here. You don't want to get the a reputation for trashing people behind their backs before you even step on campus. And Lord, if your teammate finds out, your remaining time in HS will be hell on earth.
Also bear in mind that you're going to have college teammates who aren't your favorite guys, no matter what you do. If there are 34 other guys on the team you can bet some of them you won't like so much. In the end, you don't hang out with them, and that's that. It's a lot different from HS, where the rosters are only half the size of college teams. So it's not the end of the world if your guy ends up with you.
On balance, then, I don't think that you should open your mouth if you just plain don't cotten to the other guy. If you know of serious, concrete and proven issues, I think your coaches might well appreciate being given a heads up. E.g., serious and proven problems like drug use, alcohol use, stealing, etc. If you choose to say anything, just be as deferential as you can be, take care not to come across as too eager to trash the other guy, keep it along the lines of, "I am not sure if this is my place to say, but I just feel like you should be aware that ...."
It's playing with fire, and in most cases totally unnecessary, so I would definitely be sure I had concrete proof and I was dealing with a genuine and very big problem before I even thought about bringing it up. Beyond that, the fact that you might not seem too excited about the other guy might tip off your future coaches that not all is right with bachelor # 2.