It depends on whether your cards are linked to the same account or if they are different accounts. Cashback is calculated for each account and all cards associated with it. For example, if you have one account and two cards linked to it, then all purchases made by both cards will be totalled and the cashback will be paid to that account. But if you have 2 accounts and 2 cards, each linked to a separate account, then the purchase amounts of the different cards will not be totalled.