Me and my brother just figured this out yesterday.. you have to have one wired connection for one PS3 and one wireless connection.
Go to Settings/NetworkSettings/InternetConnectionSettings. Click Easy/(one Wired Connection, the other Wireless)/then click Automatic settings for the remainder of the pages, Do not use proxy server. Note: If this does not work, then on the first page, click Custom instead of easy and follow the same steps.
Then perform an internet connection test, if both PS3 work then they should be able to connect simultaneously in COD MW2 or any other game. We played for ten hours yesterday and it was superb to finally be able to do it. We tried changing the IP address several times with no luck while both having wired connections.
I think you can also do it that way but I'm not a techie. Hope this helps.