"Fortunately, the combined contribution amount is below $7,100."

?

You already told us this: "When I report both plans, the sum of the amounts on both form's line 3 exceed the maximum family amount of $7,100 for the year."

The Family Plan entitles them to the Family amount, and there is an over-55 additional $1,000 for the person with the Family coverage HSA. Or: "Married couples who both are over age 55 may each make an additional $1,000 contribution to their separate HSAs."

That is why it is based on Accounts, which are individually owned.