Depends on what kind of tuner. If you are talking about a CableCard tuner, then no, the only way is to use Media Center, and that can only "watch" one tuner at a time. You would have to actually set the other tuner to record the other show, it's not constantly recording both tuners like a typical cable box does.
If you are talking about an ATSC or QAM tuner, you may be able to use two different TV programs at once. For example, my Hauppauge card came with it's own WinTV program, I could use that with one tuner, and WMC with the other.