There is not a player mechanic for that. One cannot change allegiance of the last city, region, or duchy. I do not believe ruler exchange would work for the last region. One cannot buy the capital region or the regions bordering the capital region.
Delvin or I can facilitate the last region and associated nobility of that realm transferring over, once the players have moved the rest of the regions over. It does depend upon the numbers of nobles in each realm, the rulers, and the broader assents in the merging realm. Please have the two rulers get in contact with us.
