Rule of thumb is never to mix antenna and cable signals on the same coax. It's like putting republicans and democrats in a room and asking for a quick solution to a critical problem.
Not only do you cause your tuner to go nuts, you potentially could cause localized issues which could get you a knock on the door by the cable co. The best way to combine those is thru separate inputs to the TV (Tuner 1, Tuner 2, etc) or to use something like multiple or a dual input HD Homerun, which you may know of since you have an HTPC.