Many ways, you could use FTL http://www.dual-boxing.com/showthread.php?t=10968
You could macro an action bar switch http://www.dual-boxing.com/wiki/inde...st_using_Pages
Assist without focus http://www.dual-boxing.com/wiki/inde..._without_focus
Focus Live toon http://www.dual-boxing.com/wiki/inde...ocus_Live_toon
Focus with modifiers http://www.dual-boxing.com/wiki/inde...with_modifiers
Assist via party assignment or focus http://www.dual-boxing.com/wiki/inde...nment_or_Focus

I personally use a /focus (toonname) on my window swap keys, and /assist focus.