The credit limit of the main card is shared with the credit limit of the auxiliary card. When swiping the card, the credit limit of the secondary card is not enough. Where's the main card? Is there a credit line available?

If the credit limit of the primary card is 20000 and the credit limit of the secondary card is set to 10000, then the two cards share the credit limit of 20000. If the primary card uses 10000, the secondary card can use 10000. If the primary card uses 20000, the secondary card has no available credit limit.