If you have a the DL Platinum Card cert, it can only be used with Main Cabin fares (and not C+). Any upgrade offers will have to be paid on both seats, and not just one. The upgrade offers can be either more or less than the the fare difference from coach to First. There's no way to get the upgrade for the fare difference when booking with a Cert if it happens to be less than the online seat upgrade offer. If you have the DL Reserve Card Cert, you will almost certainly be better off booking the companion ticket as FC rather than hoping for a decent online seat upgrade offer (which, again, would be applicable to both seats, not just the primary).