Ok, let's look at a simple setup to get you started:
Building 1-> Router-> AP w/ HighGain External Antenna->
--- ->Tree-> --- ->
AP w/ HighGain External Antenna-> Router-> Building 2...
First, note the High Gain External Antenna. You will be needing 1 of them for each AP. :? Second, both of the AP's have to be in Bridge Mode. So be sure to find an AP that supports Bridge Mode.
I'm not the man with the most knowledge about Wireless in this Forum, but i hope that this might help you on the right way. :wink: