If this secondary card has the same card number (PAN) as the primary card it is not possible to add this secondary card to your Curve wallet, too. This is a known issue.
Also, if the secondary card does have a different number than the primary card, but if it is already added to the Curve wallet of your wife it is not possible to also add it to your Curve wallet. This is even not possible, if the secondary card is removed first from your wife’s Curve account. Also known issue.
If the secondary card is not in your name, it is not allowed (even if it is has a different card number) to add the card to your Curve wallet. Linked cards in your Curve wallet, must be your cards, not a card of someone else.