How about this: on the page to offer partnership to someone ( https://secondlife.com/my/partners.php ) let's add an option to offer to share your last name with your partner. If your partner accepts the proposal, their last name will be replaced with the last name of the offering partner.
The partner having their last name replaced will keep this new last name as long as they remain partnered and will have their last name fall back to their previous last name if and when unpartnered.
Using this option will add extra cost to the partner offer, about the same as what is paid to change last our own last name. Will be paid by the offering partner.
At any time, any partner can decide to break the partnership. The one who does the break will have to pay as much for having their previous last name back or they can select an cheaper option to break the partnership and revert back to a regular Resident last name.