If you have nat type open on mw2 you can go on mw2, play like 1 min of a game, then take out the disk and insert bo2 (switch fast, but not to fast) also sign out when you switch the disks so when you get into bo2 you sign in and you should have nat type open
This does not work for everyone tho, but i know many pl that it works for, included me
Hope this helped