You certainly can do what you are asking no matter what mode you are in.
Mine is set to dual mode and I have tv2 as the prefered tuner to record to (it's in the menu somewhere, but I'm not sure if it's record plus or what). When I set a single timer, it defaults to tv2 and I can watch any other channel I want on tv1. If I set 2 timers for the same time, both tuners are being used, but I can still surf the OTA channels. This is setting timers for future programs.
When I'm watching the program live and hit the record button, I get a screen asking which tuner to use. If I don't select tv2, it will use tv1 and cause the conflict you are describing in dual mode.
If in single mode, after starting a recording of a live program, just hit pip swap to change to the other tuner and surf away.