Scott, if you are on two different servers, it is difficult to do this. Gobigtime has code that will allow it by tossing password files between two servers. Costs way too much money, though.
I already have a script coded using mod_rewrite that allows sharing, used it for years with relative success. Worked great until ISP's started filtering out referring headers, had hahahahahahahahahaha messups, and had to unprotect multimedia files so they would stream.
Check this solution out. I am going to try this script out down the road. It is only $149, and does much more than allow the sharing of members areas.
http://realtimescripts.com/docs/investment_guard.htm
Read that page closely. I have never seen something that seems that good.
If you can't afford that, then get a dedicated server with your buddy, and link to eachothers content through symbolic links, which works transparently and flawlessly.