Laredo and a couple other Keystone products did, in fact, connect two rooftop units to the same ductwork. The way they did it is called "racetrack ducting". There were (may still be available in current models) two ducts, installed in a "circular" fashion. By doing this, one unit is actually "pulling" air toward itself while "pushing" air to the other unit and the unit at the "other end" is also doing the same. This allows them to work in unison, rather than to oppose each other.
You can see an example and an explanation in the 2015 Laredo brochure which can be downloaded here:
http://www.keystonerv.com/brochure-archive
I've no doubt there are other ways to effectively install a dual rooftop system using the same ducting network, but considering that Keystone has already done this, using their ideas might save some of the errors in "trial and error installations"....
ADDED: For me, at least the way I visualize the A/C system, installing two separate ducting systems, one for the front of the trailer and one for the rear of the trailer is a significant limiting factor. In that type installation, there's no cooling in the front unless that system is on and there's no cooling in the rear unless that system is on. So, on "mild temperature days" the system would be limited to requiring both systems to evenly cool the trailer. There's no "alternative means" to reduce noise levels by using the front system when living in the rear of the coach and no way to use the system to cool both ends of the trailer when using a generator of "normal size"... Definitely, at least for me, considerations that would impact installing two separate systems in such a small trailer.
Just my thoughts, based on how we use our RV and certainly not any criticism of how others plan to use theirs.