Sounds like it is just meant for situations where the primary unit can't keep up, which means you may never need it. But that is a big camper. Congrats on the purchase!
Since the thermostats are together, you can't set separate temperature zones.
When you first set it up at the site, you can crank them both up to cool it down faster.
It might be awesome if the other one ever fails, though!
Only downside is, you probably have a 50 amp connector because of the 2nd unit. You could always just use an adapter (I assume) and just not use the second a/c unit.