I agree, and this solution possibly (probably) solves both problems.
For people wanting a traditional joint account, they transfer all money into the shared account (either by standing order after payday, or alternatively by paying into it directly if it has its own account number), and then set the default account on their card to be the shared account.
For people wanting a shared and personal account they just don’t transfer everything into the shared account and switch when necessary.
In this scenario marking certain known payments as “always shared” would be a nice addition although not strictly necessary (i.e. I know that any payments to my mortgage provider are always shared etc.).