Yes, I have both and both are active. No charges for both.
2 scenarios:
1. If you got Exclusive Mastercard DC with welcome kit, then simply apply for a new VISA Signature DC from App's side menu>Apply Now>Debit Card. This is not an upgrade, but a request for another DC.
2. For someone who got Visa Signature DC with welcome kit, the process is a bit different. Because Mastercard DC is not available to apply from app, we need to it apply from calling customer care. KEEP IT IN MIND...NOW THEY ARE NOT ISSUING SECOND DEBIT CARD. No matter what you say, at what time you call (I tried for a week, called different times at day, night, 1am, 3am, 5am all but same response). They will say that they need to hotlist the first DC to apply for second DC. Simply accept that. And Let Visa DC to disappear and MC to appear in your app. Then follow scenario 1 to apply for new Visa Signature DC.