Two separate drains are installed, especially on longer trailers, because the distance to join the two together is simply too much. Often times, one is forward of the axles and the other is behind. Getting enough grade fall between the two, working around the main frame beams and all the lesser beams, working around furnace duct work and wiring is simply too much of an effort for the manufacturers.
If you decide to do some self-help plumbing and connect the 2 together, these are the obstacles you have to work around.
Now, you can easily drop the connecting pipe under the cover for the underbelly and have it hanging to clear all those obsticles. But, when driving, there is always the risk of that low hanging pipe hitting something, or something hitting it.
And in cold weather (below freezing), if that pipe is not enclosed in the underbelly, it will freeze up with fluids and such in it. So, in order to do it right, it really should be enclosed inside the underbelly. Otherwise, hanging below, folks would be complaining big-time about what a stupid set-up that is because the pipe is exposed and getting damaged from road obstacles and freezing in cold weather.
So, over all, it's just easier, and the law suits are negate if the simple solution is to connect with 2 valves. THAT is the reason for the 2 valves and why manufactures do it that way.
Now about hook-ups. Here's the easy solution. Of course, Winter camping is an entirely different ball game:
Our current fifth wheel. We were set up this way for 2 months in the North Carolina mountains this year.