I don't know all the specifics about your floor plan, but my guess is that your problem is one of stratification of air. Assuming your central air conditioner has the required cooling capacity for the entire A-frame, you may be able to make the conditions tolerable by promoting vertical mixing using ceiling fans. You can run them in one direction in the summer and reverse the direction in the winter. You can get a pretty decent fan for less than $200. I'd give that a try before spending a lot of money on a second system.

Thanks Charles. We have a large ceiling fan up there but didn’t think about flipping the direction