1. get your main and your alt into the group,
2. move your alt to galleon and log out here
2. log on your main and pull the boss.
3. at ~20 - 30% hit alt+f4, log on your alt (SSD helps a lot, or you can turn all your addons off on your alt) and get him flagged for your alt as well.
after hitting alt+f4 you have ~1min time until your main disconnects, you have to kill galleon before your main disconnects.
be sure to use alt+f4 and not just starting 2nd wow and logging on your alt there, your main will be instantly disconnected if you don't use alt+f4