I had 16 unsuccessful calls to get Master Card Debit card, every Customer care person was knowledgeable & said that Only One DC is allowed, system would allow to take a new Debit card request only after blocking the current Debit card. I was lucky the 17th Time and got a Customer care associate who raised a request for Master Card DC again.. I just got Master card delivered today..
So my Guess is that the System is allowing to have 2 debit cards and its not fixed to have only one Debit card but the Customer Care associates have been instructed to state that only one Debit card is allowed to cope up with the system Gap, few of the customer care associates aren't informed may be and i got one of them to help my request...
I would say keep calling till you get the wrong (Right) Customer service associate who can get you the request without blocking the card..