<HTML>Anthony:
Sorry this took so long. I had some important family matters to take care of.
You can find the plugin <a href="http://www.redrubberball.net/del/files/StrokeIt_MarkKill_1.0.exe">here</a>. It's pretty self explanitory, but what you're going to want to do, is set up a gesture activate the previous window, and then mark the new one. In effect this creates a gesture for an exclusive two-window alt-tab.
Good luck! :)
-Scott</HTML>