i have two displays, one above the other, and set up that way using xrandr. when maximizing windows, the problem is that only heights of horizontal panels on the virtual screen edges are taken in consideration, but if horizontal panel is placed at the top edge of bottom display or at the bottom of top display, the maximized window ignores height of the horizontal panel and is partially placed under the panel. steps to reproduce: 1) set up two monitors, one above the other 2) on the bottom display, place horizontal panel at the top of the screen 3) open some app window and maximize it (it will ignore the top panel which will overlay it) the same applies to placing horizontal panel at bottom of top display.
Struts don't work "between" monitors on the same screen. That's part of the spec, see http://standards.freedesktop.org/wm-spec/wm-spec-latest.html#id2552096. As you can see, they are set from the edge of the screen, not the monitor.