My first panel is getting crowded, so I created a second one just to hold the Window Buttons item. Unfortunately, the second panel (which stacked right next to the first one, vertically, on the right of my screen) doesn't push windows to its left. I tried toggling the "don't reserve space on borders" setting on and off to see if it would trigger, but no luck. It didn't work any better on the left side of the screen by itself.
Using Ubuntu 12.10 (not Xubuntu, Ubuntu with Xfce), dual monitors.
OK, I managed to get this done by setting the window manager margin on the right to 96 -- 48 for the second panel, and 48 for the first one. The first panel worked fine with no workspace margin. I'd prefer if the panel did it automatically (so I could move it around, etc. without having to worry about changing them) but at least it works.