The video is wrongly named (it's made by a german guy and he translated badly), it's multi pivot, not steering axle. If you watch the video you can see the double has no steerable axles. It's what you call a B-train, and functions as it is supposed to. They are allowed in the USA along predefined routes, Some places and routes even allow triples. Here's a nice 17 axle resource hauling double from South Dakota that can gross out over 170.000 lbs:
The one I showed the video of is a MOD btw, the doubles that SCS software showed us are the stubnose type you have shown (STAA double), those aren't ingame yet.
Names of doubles in the US:
Map where doubles are allowed (bit outdated):
The extra dolly in the heavy load DLC is load bearing btw, the last dolly is connected with one of those rigid sideswing pins, just like the steering point of an articulated loader. It is called a spreader-stinger in the USA.
Here's a more detailed picture of a small spreader-stinger, where you can see the turning pin construction. The pistons are used to regulate the amount of weight that gets transferred to the stinger.