When a cavalry model with a lance charges a high defence enemy, say, a rivindel knight charges a Nazgul on foot, how does the plus 1 to wound rule work when the knight needs 6/4's to wound, does it receive that bonus on both roles or one roll only ??

Thank you in Advance !!

In these cases both rolls add one, so the roller needs 5 on the first roll then 3 on the next