I see each gpu goes to a different monitor and deals with whatever clients you tell it to. Can't use two gpu with single client or with single monitor. And switching from main to slave (assuming on different video cards and monitors) isn't going to be as fast as people would like, even at same resolutions.

Still I guess for 5 boxers on a single cpu, one gpu/monitor for the main and one gpu/monitor for the 4 tiled slaves is optimal.