How do you play PS3 online split screen with one PS3?

May be...

Me and my bro want to play online with his PS3 in online mode instead of 2 players offline.

You can play two or even 4 player split screen online but you need the right game(s) two that I'm aware of that let you are Warhawk and Haze.

You can't play two players online at the same time on the same PS3 console on most games. If you want to play split screen you will have to play offline unless you have the games that are compatible with split screen online.

Sorry you cant but i wish you could you might be able to if Sony bring it out on the next update.

only Warhawk and Haze lets you do that so far

depends what game you want to play alot only do single player online on the one console