First of all....if you can build like the quality THAT one was built of....go for it.
I've seen those done on here before (can't recall where) and it always made sense to me, but if I were to play devil's advocate and pick it apart, I'd say that your issues may be:
1. The amount of work may outweigh the benefit of just doing a "standard" UDS.
2. I'm not so sure that the grate area is MUCH greater, if at all, than two 22.5" grates on a 2-rack UDS.
3. One reason that the UDS works so well is that you're getting both direct AND indirect heat. With that one, it looks like you'd have more indirect than direct since the bottom of the top barrel would diffuse the heat quite a bit. This isn't necessarily BAD, just different.
4. IF you did build one, I'm wondering if you wouldn't benefit from some sort of smoke/heat diffuser/chamber at the bottom of the upper barrel to more evenly disperse the smoke and heat throughout. This would prevent hot spots (which MAY not even be a real issue. I'm only speculating.
5. If you have easy access to barrels, you COULD cut the top off of one and use THAT for a lid for a UDS, then you could use a 2 rack design and still be able to put big stuff on the top rack. OR...you could also make a top extension on a UDS and have 3 racks too, which would bump your potential capacity WAY up.
That thing looks bad azz. Since I already have a UDS....I'd sure build one.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team