This has probably been posted before but it's really handy, and I've seen this question in the emulation thread a lot. Simply create two accounts, get on both of them at the same time with two separate WOW instances, invite one, convert to raid, and then click the 'x' button to close the one you don't want. Doing this causes the game to treat the unwanted account as a D/C and the raid will stay, granting access to 40 mans and such.
hope this is helpful to people.