Trying to do two exposures one for sky and one for buildings, but once I export one exposure whats the best way to go back and change the whole project for another exposure. Then I can go back in and garbage mat the buildings and have a good exposure on the two elements

Please use the search feature and search for "Snapshots" - that's the feature that allows you to save the metadata for the whole sequence and do a separate editing, and come back to the first later.