Any Problems Mixing Trio AP For Roll and Dynon For Pitch?

I have a single axis Trio AP installed. I also have a Dynon D180. I'm thinking of adding pitch by simply adding a Dynon servo and letting the Dynon drive it. So, the Trio would control roll and the Dynon would control pitch. This would I think be a cheaper and easier solution than upgrading my Trio.

Are there any negatives to doing this that I'm missing?

no problem.

I would suggest you figure out how to wire a single switch/button which will disconnect both autopilots.