The Scientific Atlanta units that cable companies use have this but it's a rather poor feature in their implementation. I find, with Tivo, that I end up using it differently. Instead of trying to watch two shows at once, I switch between them. I watch program A on tuner 1 until a commercial break, then pause it. I then switch to the other tuner and watch it til a commercial break, then pause, switch tuners and unpause.
This is much more useful for watching two games at the same time, since you get to see all of each game without being distracted.
With the SA units, you have to use PIP to change tuners, which is ludicrous.