The goal isn't to program two at once really, I have a stripboard 328 in a robot and I want to program it cheaply, so I don't have to fork out for an FTDI cable or keep taking the chip out and putting it in my duemilanove.
So I was thinking, could you hook up a ground, VCC for both sides of the stripboard chip, hook up the reset buttons and the crystal pins and serial lines together and when you upload one sketch, they would both by in sync and both be programmed at once? I was thinking of a blank DIP socket with wires coming from it that could piggyback on the 328 in the duemilanove when I wanted to program the other stripboard 328.
I'm not going to be programming it millions of times so I don't mind the host 328 getting reprogrammed along with it, even if it is a waste. It saves me taking any chips out and breaking pins on them which I've already done once.
I don't know if this would work though. I can't see it not working without the duemilanove 328 in the 'host', but with the 2 chips, would that work?