"select to pick where the other side is built (so that you don't actually have to build 2)" sounds great, even better than my original idea  
 
treat it as a support tower and u wont have cloning problems 
 
the same idea can be applied for other turrets, like an acid stream between the towers for a fire/water combo, lava stream earth/fire, or even a knockback wind wall ( light/darkness?) 
 
 
 
ps. is the prism tower from page 1 already in the game? since i cannot find it, or is it already out again or just not in the sc2 version?