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.