Take a look at this ad...it says " an air conditioner that will cool the dressing room or can be diverted to cool the horse area."

http://tacktrader.com/show_item.php?tack_id=366600&share_this=Y

Any idea how you would do that?

I'll guess they are opening a door between the DR and horse area....?

You better believe they ain't doing a whole lot of cooling down of the horse area. That roof mount AC is taxed just cooling down a 7 foot dressing room. If it's 90 degrees or better with a high humidity, it will have a hard time cooling much of anything. I'm thinking they are using that "or can be diverted to cool the horse area" as a sales tactic....