I've tried doing multicrew with a fellow pilot and encountered a somewhat odd problem.
First of all, we both have FSX Acceleration and both have the latest Maddog 2010. Also, both of us set our computers as DMZ through our routers (so that all firewall functionality are bypassed) and we've both disabled windows firewall. Early attempts were made through the regular network while later attempts were made through Hamachi and both exhibited the same symptoms, and they are as follows:
When I start a session as PF, the other guy successfully connects as PNF and everything is synchronized. We were able to do a traffic pattern with FMS and everything, and no problems existed.
Once the other guy assumes PF duties (by clicking on the clickspot), after some time my aircraft stops following his and I end up on a totally different position.
If he starts a session and I connect as PNF, my plane never moves when he starts his pushback.
So, the entire time - when I start the session as PF everything works fine, and when he starts as PF it doesn't work.