Running both graphics cards Simultaneously

No way at all. You can't have two controllers conflicting, both trying to run the display.

 

NVidia says it should be possible under Windows. But it doesn't work. Nobody knows why.

 

I think the answer you are looking for is that NO, Mac OS X 10.5 does not support using dual SLI or however you call it. Windows does support it, but running on the MBP isn't really the same as not running Windows the same through BIOS. Read about this awhile back. I think that article said that even Snow Leopard may not support it either. I think article was on AppleInsider??? Could start there, but just going to confirm it doesn't work.

Good luck.

 

Even if Snow Leopard did allow it, I'm sure it would draw a lot of power thus making it "useful" only when plugged in. I mean the 9600M GT is already bad enough on battery.

 

This is where apple steps in with built in battery's across the board ;0

 

The setup you describe is called "GeForce Boost". It's basically SLI with the integrated graphics as one card. But.....SLI-ing non-like cards makes the more powerful one downclock and disable cores to match the weaker one. Since the 9600M has double the stream processors as the 9400, the end result would leave you with the same GPU resources you had with just the 9600, except they are now split across two dies separated by a PCIe link, and half of them don't have discrete GDDR3 anymore. Hence the reason the 9600M GT doesn't support GeForce Boost, as it would actually reduce performance.

 

Apple or NVIDIA would need to write drivers to enable Hybrid SLI (which still isn't both at the same time, but rather the 9600GT would kick in when more power is needed and then fall back the 9400 for regular tasks.