The longer the AC runs the more efficient it is in conditioning the air. Two smaller AC units will be more efficient than a single large one that will power on and then shut off and have a shorter duty cycle.
You also have a backup should one unit fail for any reason which in hot climates could be a life saver.
Historically the people building RVs and houses have been ignorant of AC systems and have put in twice as large a unit as is actually needed thinking that overkill was the prudent approach. Actually with the oversized AC units people were less comfortable thanks to the short duty cycle. More power is consumed with less conditioning of the air actually taking place.
