Why does it cost the same money to swipe one of the two CCB credit cards as to swipe the other?
The credit card limit under your CCB name is shared rather than accumulated, so using one card will affect another card.
Suppose you have two ab cards with credit lines of 10000, then the maximum amount you can overdraw is 10000, not 20000. A card costs 5000, and B card is useless, so the available quota of A card is 5000, and that of B card is 5000.