Interesting. I have limited experience, but have never seen a small trailer set up with separate 3" drains for the gray and black systems. Having 2 drains, one on either side of your axles/tires is going to complicate the dumping process. As you note, you will need to set up two drain hoses with a Y connection for hookups. This always poses problems as you try to "walk the drain hoses dry". Your dump station time will increase. And you lose the ability to fully "flush" your black drain system with your gray tank water. I would consider this system a minus in a small trailer. They probably did this under the guise of "weight saving"...but how heavy is 10' of 3" plastic pipe? I'd guess it was more a matter of "cheaper".
I do have some experience with this issue. Our 40' toy hauler with 2 bath rooms has two separate drain systems, but you sort of expect this with the really big rigs. And yes, it is a bit of a pain to deal with...but in our case, it is worth it for that second bathroom!
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