While Watching a Show:
Press RECORD to record the program you are watching.
Press SWAP to switch to the other tuner and find the second program you want to record.
Press RECORD when you locate the second program you want to record.
From the Guide:
Highlight a program you want to record from any listings screen.
Press RECORD to set up the first recording.
Highlight the second program.
Press RECORD to set up the second recording.