Been thinking 2 ipads to mix music with could be interesting. However where would you store all those tunes? Would you put the mixer on a third device or even a iphone/ipod touch? Could you run the whole thing on one ipad?
There are a few stories exploring the idea. But generally most of the problems related to the ipad are also related to the iphone too. Things one audio output, no ability to add an external usb soundcard, locked up music subsystem, etc, etc. I did however see that the ipad has already been hacked or jailbroken, so who knows what might change for a serious dj setup.