Why don't you stick them within a few feet of each other and turn the transmitting power down for your test? That would seem to make the most sense to me.
Or, if you wanted to get fancy, figure out how far the buildings are apart, calculate the loss DB of the air inbetween them. Do some research, figure out what material (ie glass, wood, whatever) has a similar loss db. Build an enclosure out of the material. You may need to only build one enclosure depending on the distance between the buildings. If you need additional signal loss, build another...I think you get where I'm going with this.
-brad
www.ccbootcamp.com
(please rate the post if this helps!)