The TiVo does not have a cable tuner in it. It records
the RGB output from the STB. The result is that if you
want to change channels you can't record the one you
were watching.

However, what you can do is watch a recording on the TiVo while recording another channel on the TiVo. As the STB only outputs one channel on RGB you and the TiVo can only access one cable channel at a time.