I'm kinda interested, why error message always says "they are..."
It used to be like this before August 2010 wipe. It was either 2 or 3 people that could talk to a merchant at the same time. I blame TLA/SDK engine - in TLA server and other SDK servers, you can only talk to one person at a time.Unless it's intentional, which I don't see why it would be.
Yes, this is enforced by an engine change, will be changed in the future when possible.