In addition to welding, I do quite a bit of heavy haul lowbed work, and assist the local tow companies with big rig wrecks and recovery, I've done this exact same thing many times. If the body structure of the trailer is still intact, that is by far the safest, fastest, and easiest way to move them. Most, not all but most, trailers with sliding axles only have two, 1" pins that hold the axle group and suspension in place, so there is a lot more holding it onto that tag trailer than there would be when it's normally going down the road. If they are damaged too badly, we will put them on the lowbed or a full size flatbed, but that is a last resort. On a lowbed, that thing is going to be hanging 20' off the back of the trailer, on a standard flat bed, it's going to be way over height. And in both cases, the entire load would be relying on the tie downs. By doing it this was, the front half of the trailer is going to stay hooked to the truck no matter what happens.
Speed is important with clearing roads, with this method, I can hook the tag trailer to the pintle hitch on the back of my truck, meet the tow truck on scene, the tow truck lifts the back of the trailer, I back the tag under an tie it down, then unhook, drive around to the front, hook up like normal, by this time the tow truck has hooked to the original tractor and we both drive off and clear the road. The trailer is ALWAYS emptied on site, or pulled down to the next wide spot or exit to be emptied, the are never hauled loaded.
If you think this is bad, you guys would freak out if you saw how we haul bridges, beams and structural columns!!