I have the same problem. Here in Florida it gets hot in the summer and the two 13K BTU units I have run all the time and just make it cool enough. There is no margin so for me a 3rd unit is an option. Thought about replacing them with 15K but you only gain 4K of additional cooling. A 3rd unit give me 13K and some backup. Don't know about your wall problem my center vent is dead center of the hall. The biggest problem is routing power to the unit. Will have to figure out what phase one AC is on with the Micro and use the other for the third AC.
