They actually, unless they TiVo it, can't watch both.
There's actually 3 CSIs, and so there's new episodes on Mondays, Wednesdays, and Thursdays.
BTW, before TiVo, there were these things called VCRs. They work kind of like TiVo, except you didn't have to pay a stupid monthly fee and you could actually eject the cassette and take it to other rooms (or even other houses) and watch them.
Ask your parents about them.