If you are recording two programmes then you may be limited as to what other channels you can watch due to the way the channels are grouped together into six MUXes. Each tuner can only decode one MUX at a time and if both tuners are already busy with the recordings then you will not be able to change channel to one of the other four MUXes.
Recording two channels from different multiplexes you can view any channel sharing a multiplex with either recording
Recording two channels from the same Mux you can view and time shift any other channel. In fact once one of the recordings has finished you can record what you were watching from the time shift buffer as well. In effect recording 3 simultanously transmitted programmes.