OK, after my rather blunt response earlier, here's the longer reply.
First off, these back scabbards are completely ahistorical. There is not one single reference I know of in the entire body of medieval art, depicting such a device. And I have a reference library for these sort of things in excess of 10,000 manuscript images, a few thousand effigies, hundreds of digitised books from all over the whole of Europe.
Not one reference to anything of the sort.
Technically, in terms of performance, they are utterly useless. A scabbard is designed for three uses - in order of priority:
1: to protect the sword from corrosion, by being exposed to the elements. In reality, a sword will spend vastly more of its life sheathed, than being used. The sword in that picture is a modern wall-hangar, a lump of corrosion-resistant stainless steel. Real swords are carbon steel, which corrodes rapidly - quite literally, leaving one out in the rain for an hour will leave it with marks on the blade. just one hour is all it takes.
2: to safely protect the blade - from accidental contact, (you don't want a sharp edge exposed anywhere its going to get caught, accidentally run fingers along edges, etc) and These exposed sheaths fail on both performance criteria of a scabbard.
3: it should assist transportation. This comes a distant third in importance.
the design fails utterly on the first 2, and #3 is only in the context of it being carried.
Its construction method is completely wrong. Chrome-tan leather, no wood, no linen, steel chromed buckles. these are not medieval materials, and it shows. Its an entirely modern design "solution" to a problem that was never there in the first place. To my eyes, that's as out of place in a "medieval" fantasy setting as an AK-47. would be.
The next thing is, even with the hideous oversized boxy construction, these things simply don't work. they're designed for gullible renfaire twits who think they're "scawttish" wearing their polyester kilt to wander around with a sword on their back and blue face paint like a fucking smurf. (I apologise for the language. No, fuckit, I dont apologise for the language, these gobshites are an affront to my nation.) - they still cant be drawn conveniently. they get in the way, you cant sheathe them easily, because you cant see where the tip is going.
They're don't work, because they're made by people who have never, once used them in reality. Medieval scabbards work, because they were the product of real-life experience, constant development and honing of the systems used over generations of use. These scabbards are akin to me, someone who's never flown an aircraft, designing a fighter jet. It wouldn't work, because I've not got the experience to know why the throttle should be in one place, or the fuel gauges in such-and-such a place.
Nowadays, we don't need to use such kit, its only ever going to be used to carry it around in a ren-fair. They're trying to invent a solution where none was ever needed
You have a big sword, it went on the horse. Or in a cart. simple as that. Because you didn't get people walking around for days on end, on foot. We domesticated the horse 5,000 years ago, it was ubiquitous. Every army in Europe would've had a baggage train for the transport of kit. You didn't traverse the country wearing full armour, carrying a spear, a pole-arm or a great-sword, plus your side-arm, your dagger, your purse, your food, your spare boots, your bedroll, your knife, your shaving kit, your rosary, your spade for digging the latrine, your firedog, your cooking-pot, the bricks for the oven to bake your bread, a dozen stakes for the palisade wall....
And so on. You stuck it in the baggage train. Or if you weren't in an army, you went on pilgrimage, you had a pony, it carried your stuff. You had to travel, you used carts, wagons, horses. Designing "solutions" like that scabbard is like designing a "solution" for the problem of every solder in Afghanistan being able to transport their personal M252 mortar up mountainsides, completely oblivious of the fact its stowed in the vehicles they get there in - not because of weight (a good 2h sword weighs less than 3kg, with a scabbard.), but simply because of the bulk. they take up a lot of space.
Lastly, I'm sceptical that you even NEED a back scabbard of any sort for most two-handers. I've used and worn plenty of swords in reenactment and living history. A two-handed sword might be 1m 50cm to 1m 70cm or so (60-70ish inches.). Only the truly massive, heaviest examples are bigger than that really. A longsword that can easily be carried on the hip, is 1m 30cm, with the tip 10 cm above the ground, and the pommel forward. adjust the strap lengths, modify the suspension system to pull the sword a little more horizontal, and I can see a 1m 50 twohander being possible to wear on the hip.