In single mode, all outputs are the same and both remotes operate tuner 1, the current tuner. (Swap can be used to switch tuners but the active/foreground tuner is always seen as tuner 1.)
In dual mode, remote 1 operates tuner 1 which is seen on output 1. Remote 2 operates tuner 2 which is seen on output 2. Tuners cannot be swapped. In dual mode, remote 2 can be used to operate tuner 1 (as described above) but control of tuner 2 is lost unless a second remote 2 is available.
The advantage of using remote 2 to control tuner 1 is being able to connect one of the HDTV outputs to a TV in another room and then control tuner 1 using RF. That can be accomplished by switching to single mode as well as reversing the tab.