Generally speaking, if the "G(or D)+" has a strong preference on the desired candidate, but the "D(or G)-" has a weak preference on its top-1 incorrect candidate, then this combining method would correct most "G+D-(also G-D+)" errors.
the advantage of combining two models would vanish if the "G(or D)+" has a weak preference while the "D(or G)-" has a strong preference over their top-1 candidates.