Actually, he doesn't. It specifically states that he takes up one of the HQ slots, not the Inquisitor Lord HQ slot. Although I am sure it was just an oversight by the writers, the fact remains that both can be used in conjunction. This also applies to Brother-Captain Stern and a Grey Knight Hero. I wouldn't recommend it though, first off it'd cost alot of points and second you'd probably get yelled at by a few of your opponents. But if you want, I say go for it.