Having one secured credit card may be enough to help you rebuild credit. After all, rebuilding credit doesn't depend on the number of cards you have, but how responsibly you use the card.
That said, there’s no harm if you get an extra one from a different credit card issuer. For instance, you can get one from Amex, and another from Discover.
As long as you can pay your monthly bill on time or in full, having two cards can help you boost your credit score faster. Plus, you'll still have an alternative in case you lose one of your cards or fall out of favor with the issuer.