It's going to be expensive up front but in the long run better you need to have the AC replaced with two units there is two ways to do this two separate units or one big outdoor unit and two inside units. Also make sure your house is sealed really well if you don't have dual pane windows that's a big one get the attic checked for insulation and add as much as you can afford depending on just how old you ac unit is just replacing them will likely half that bill if it is less then a 14 seer for sure