If you go with the deposit only card option, you could adjust your feed to display the aggregate of the two cards; but you're effectively capping an individual

If you go with the deposit only card option, you could adjust your feed to display the aggregate of the two cards; but you're effectively capping an individual transaction to $20. The donor therefore sees their changes reflected in real time, but any cheats are limited (hopefully a cashier would get suspicious / frustrated if someone tried to work around the $20 limit by going multiple times in succession).