Perform mode multiple monitor index

Hello to all,

I searched forum a bit but couldnt find any answers also look to the wikipage. Maybe I couldnt find.

Lets say I have 1 monitor and 2 projectors (1280x720px x 2 = 2560x720 canvas).

What I want to do in perform mode;

I want to just use 2 projectors but still be able to use 1 monitor.

I look to the window chop prefences but there is only single, main and all options.

How can I force in perform mode to use just 2 projectors?

Sorry if its a silly question.

Thanks,