The toxins of course being the biggest issue, but heres the thing.

Different requirements are easy enough to simulate in the same setup. Firebellies and Chubbys aren't that different that you couldn't provide them both with exactly what they need in the same tank given the size. Chubby's aren't the most active and firebellies tend to stick near water (at least for me). My biggest concern is the toxins but I was wondering how they work. In nature there would be constant exposure near the Firebelly home. So I assume theres something that either removes the toxins or neutralizes them. My first assumption would be plants / water. So in a 55 gallon tank designed to be long, well planted, and an artificial rain setup would toxins really be an issue considering the lack of interaction between the two?

And I also recall reading Chubby / Firebellies are compatible but again toxins take years to kill so it is hard to say if someone has had a good run for 2-3 years, but since he hasn't hit 15 years its difficult to say whether toxins have an affect on the over all life of the frog. It would take multiple setups with multiple firebellies / chubbys to see if there was any real result.

This hobby has come a long way in the last few years, compared to the 90s, despite of this we are still living on the information passed down from us by people who didn't fully understand all of the interactions and requirements.