Dual waste valves on my new(er) rig - can I combine them?

Not quite the same but in a previous trailer I had two gray tank valves. It was a pita moving back and forth and I did run a pipe between the two and tied them together. It worked flawlessly. In my research it seemed that many of the more expensive RV's did tie them together. I don't see why two blacks would be any different. I'd give it a try. It was worth it for me.