Best way to hook up sewer hoses for two holding tank?

I'm inconsistent with where I put the "Y" (aka "wye") and if I put a hose between the sewer outlet or not. It all comes down to the site layout and how it feels.

Here are the parts that I use (both of my sewer outlets are black AND gray):
http://www.learntorv.com/2013/05/my-camper-has-two-sewer-hookups.html