Is there a way to run two AC’s off of one Easy Start unit.
I’d like to eventually design a solar system for my new Airstream with 13,500 and 15,000BTU units.
Although it would not be electrically impossible to wire a single EasyStart into 2 A/C units, it would be impractical, would compromise the EasyStart's ability to monitor the compressor during steady-state operation, and very likely cost more than just buying a second EasyStart (in wiring, components and labor). This is because the cross-wiring that you would have to add between the A/C units, plus a bunch of switching devices (power relays) to connect the respective wires to one compressor at a time, would end up being a nightmare. Please realize that I'm not telling you this because I want you to buy 2 EasyStarts instead of 1. These are just the facts.
Many of our customers with trailers with 2 A/C units designate only one A/C as the only A/C they will use when they're operating on their single 2000W generator. Of course, their choice in doing so trades off the ability to run both A/Cs on a single 30A utility hookup or on 2 paralleled generators, but we can totally understand the choice made in this case since not everyone can afford 2 EasyStarts.
In your case, if you intend to setup a solar system with a single 4000W inverter (or larger) to run both A/Cs, you would stand to gain more and in fact would almost require the benefit of installing an EasyStart into both A/C units. That way, it won't matter which A/C is cycling on at any given time. The same is true for the situation with a 30A utility hookup (3600W). I did an installation into an Airstream with both a 13.5k and a 15kBTU A/C back in February and wrote a report, which is attached (please ignore the fact that the EasyStarts back then used the now-obsolete black enclosure).
Alternately, if you intend to only run one A/C with your solar/inverter system (2000W), then you'd only need 1 EasyStart and you'd have to choose the A/C that would only be powered when using the inverter as the power source.