Neil,
Usually I use the red Tahoe you see on the trailer to do the towing. I happened to have that little Peterbilt with a receiver hitch so thought that might make a better picture for you.
As to it being "a full on semi," it really isn't. This is a single (rear) axle unit. Also, this is what we call "a day cab" in that it has no sleeper. This one happens to have a fifth wheel on it but could just as easily have a 9' dump bed and hoist instead. It's really not that big of a truck. This one has a 300HP Caterpillar 3216 diesel with an Eaton Super 10 speed transmission (like the old 5 + 2's).
For pulling something like this trailer the Pete 330 is really overkill. I bought it to pull an eight place, 40,000 lb. GVW car hauler I'd bought. That's a far better match for it.
My red Tahoe handles that trailer with my Deere 4200, FEL, BH and a mower or box blade with no problems. If I were going to do a lot of towing with two vehicle as you see here I'd probably go with a Suburban K-2500 with a towing package.
As to being legal, I'm completely legal with my Tahoe so long as I have a brake controller for the trailer brakes. I can pull a tri-axle four car hauler with a pickup legally, also.
If you want me to look for any other pictures of unusual (to you) things we do here, just ask. I'll be happy to help.