New Skylake MacBook Pros coming October 27.
MacBook Pro Does Not Support Both GPUs Simultaneously
NVIDIA has since clarified that the new MacBook Pro only supports the ability to switch between the integrated and discrete GPUs for power conservation ("HybridPower"), but does not allow the use of both at the same time ("GeForce Boost"):
Apple's Macbook Pro (Late 2008) does feature both the NVIDIA GeForce9400M motherboard GPU for everyday computing and the NVIDIA GeForce 9600M GT discrete GPU for high graphics performance. You can switch between the Geforce 9400M motherboard GPU (called energy saver mode) and the Geforce 9600M GT discrete GPU (called performance mode), but you cannot use both GPU's at once in this implementation.In fact, it's been revealed that switching between the two GPUs on the MacBook Pro requires the user to log out of Mac OS X. Apparently, other implementations of NVIDIA's Hybrid SLI allows on-the-fly switching between GPUs, so it's not clear if this limitation in the MacBook Pro is a software or hardware issue.
This inability to use both GPUs simultaneously appears to be a hardware limitation and unlikely to be improved in future software updates.