There is this episode of Ice Road Truckers were they were hauling a load to Prudhoe Bay, but it was too heavy for a rig to haul it over some passes so they decide to have the rig haul it by pulling the load, but they had another truck push the load from behind, hence getting this load over the pass by some means.

But what I found interesting is that they had the pusher just keep on the gas slamming bumper to bumper, but to let go of the steering wheel as the truck in front was going to do the steering.

It was exactly hitched, but the truck pushing it was being steered by the one pulling, so I'm wondering how that exactly worked.

I guess it is like towing. If the front of a towed vehicle is locked in position, the wheels just follow. Maybe keeping the bumpers 'slammed' together keeps the wheels straight because the front end is stable. I dunno!

^What he said^
The pusher truck acts like three more axles, the front wheels don't need to be turned because they'll most likely be used like slave wheels like the ones that can be lifted up on dump trucks.

Hope that makes sense.

The force applied from the push truck over the rear bumper of the trailer is evenly distributed over the multiple contact points between both pieces of equipment. Even big trucks have differentials, which allow the push truck to stay evenly planted, parallel to trailer. Theoretically, as long as there is enough friction between the contact points, the truck will not slide side to side on the rear bumper of the trailer, thus allowing the main tow vehicle to provide the steering, and all the push truck driver needs to do is keep on the throttle to keep the push truck planted firmly on the trailer. Essentially, the push truck merely provides extra power to aid the heavy loads. The actual weight limit of the pull truck remains unchanged, though, as the load isn't dispersed on the extra axles of the push truck. That's typically why you see those loads on lowboys and trucks with 3 or 4 40,000lb. axles each.