I don't think you're going to find an adapter to hook up two head units to a single factory connector. In fact I'm sure of it.

It's time to get the soldering iron out, you'll have to start splicing. Hopefully you're comfortable with that. Good note on the power wire too, it needs to be up-to-snuff.
Altogether this won't be very toigh, but yes it will require some splicing. I'd start there and then redo the power wire if it becomes a problem. If there is one, you'll just blow a fuse, so don't worry about killing the car from it or anything.