Assuming you have a HDR FOX T2 (this is the HD FOX T2 thread)
Mod edit: Moved thread to correct forum
You can watch a recording at any time whatever your tuners are doing.
What third channels you can watch while recording two depends on what you are recording.
If both recordings are from the the same MUX you can view and time shift any other channel.
IF both recordings are from different MUX you can view any other channel sharing a MUX with either recording.
eg Recording any two HD channels you can view (and time shift) any other HD channel. When one recording finishes it’s reported that you can back up the time shift buffer on the viewed programme and record that as well. (I don’t have a HDR FOX T2)