I'm using "Mirror Tall" with dual monitors. The top half of my screens are for master windows, and the bottom half are for slave windows. I find though, that as long as I have more than one slave window in a workspace it makes the bottom half of the screen pretty unusable because it gets split evenly amongst all the slaves, and each slave window becomes unpleasant to view. It'd be nice if there was a main slave so that most or all of the "slave pane" is filled with the main slave window. Then I would have half of the screen filled with the master