The only way we have have been able to achieve this, is to have one of our immedate family (all platinum) assigned to a room the with grandma and grandpa. They were ajoining staterooms. The whole party, us and grandparents, checked in together at concierge since both staterooms had platinum guest and all got the same boarding group.
You mentioned your staterooms are already linked but without an actual platinum guest in the room they would check in at the normal queue. Not sure if that is an option to move one of your party into the other room. They can always be switched back to the original room at guest service after boarding.