I just did this yesterday without any problems. If you have not done this before, when the sign in window pops up, you then hit (control n) before signing in. This will bring up 2 of the same sign up windows. You then log in with your main character on one and another character on the other. I always go into advanced mode and pick a lobby to have the characters meet for the trade. It does allow you to trade things without getting anything in return. It will just give you a warning screen first, but you can click to accept the trade. I have had friends tell me that (control n) does not work on Mac computers, so people on and Apple are out of luck with this trick.