Almost constant running AC unit

The cost of adding a second AC depends on it you already have a 14"X14" vent cut out. If you do, It's pretty simple to haul the AC up to the roof and drop it in the vent cut out (with gasket) and tighten 4 bolts inside. If the 120V wiring is not in place, you have to run a 12/2 Romex to a breaker box, then attach either a 50amp or 30amp power cord to plug it in separate from your existing power cord. Most places have a 50amp and 30amp receptacles. Having a separate plug in for the second AC means you can have both ACs running all the time and no have to worry about power management when other appliances are on.
I put in a 2nd AC this way on my trailer. Helper on the roof charged me $100. Wire, box, cord cost me $60. 2nd AC, used, but almost new, cost me $350.