It means that Christ would not give benefits while he was on earth. While he was on earth he was our representative.
Who will it be? We don't know of course, and very, very few – so far as I can tell – commentators and divines of the past attempt to make a suggestion. That is the very wisest thing. It is beyond our understanding, but the favourite Puritan answer was: everyone! All the redeemed people who have the privilege of living and reigning with Christ. It is not any two specific people. It is all his people, have the maximum privileges.