I was playing around with some simple stuff in HMT and I decided I'd let myself drive the warthog while on the turret in Single Player. 10 seconds later I was playing the second level like this, driving and gunning at the same time. But then I noticed that the Marines were not only getting in my passenger seat, but they hopped in the driver seat, too. Sometimes I couldn't move the hog if I got in the turret, but sometimes I'd have two guys sitting in the forward seats while I drove from the gun.
So, my question is, is there any way to have the Marines driving hogs?