In another thread you asked about watching different TV's using the same receiver. If you want to record something while watching something else, the other TV, TV2 if you will, is stuck watching whatever you are recording. That is true with all receivers.
Before the "to be recorded" show starts, select it in the channel guide. Choose TV2 when prompted as to where it should be recorded. You can watch your live show on TV1. You can even just put it on when it starts, pause it and go do something else for a few minutes and when you come back you can start from the beginning. When the commercials come, you can FF right through them until you are back to watching live.