if you really have to use a different reducer because your stuck, the general rule of thumb is keep basecoat reducers with basecoat and keep 2K or top coat reducers with topcoats,basecoat reducers generally dry quicker so you don't get solvent retension in your clearcoat, and topcoat reducers stay wetter so your paint has time to flow. Using a topcoat reducer in basecoat can cause what we call solvent boil( the final finish looks like it has small pimples)