Yes, you can link a maximum of 5 Bancontact cards to one app. These cards can be issued by the same or different banks. You can also install the app on multiple devices and link the same or different cards to the app. The app does not support the cards of any payment methods other than Bancontact.
Are you using a smartphone that runs on the Android operating system and has an NFC chip? If so, you can make payments both by scanning a QR code or by holding your smartphone close to the terminal (tap&pay) for each card registered, regardless of whether your Bancontact card itself is contactless or not.