Did you look at this page on 2 page spreads? Maybe this will help.

http://www.prestophoto.com/photos/gallery/11323

Judy

"If you are creating a double page layout for inclusion, be sure to leave at least a 1 inch margin in the center where you don't place anything of importance. This compensates for both the trim and the binding. If you want to be a little more cautious you can extend the center margin to 1 1/2" like in the example below."

Using your "cautious" 1 1/2" margin example, is the total margin 1 1/2" (3/4" on each page), or is the total margin 3" (1 1/2" on each page)?