If the manufacturer's specs says so then there's no reason to believe otherwise. I do know that you can have both enabled and have monitors hooked up to both simultaneously, so happy power-tasking!
There's most likely a setting in the UEFI about whether the integrated graphics are enabled/disabled/automatic. If you can explicitly select enabled, do that. Otherwise it's going to be controlled by whether or not it detects a monitor plugged in to one of the mobo ports at the time the machine is booted.