- Clear the two existing accounts (if present) in XUMM: go to Settings - Accounts and remove the account belonging to your primary (destroyed (inaccessible)) card, and remove the back up card account (if present)
- Import your back up card by adding an account (Settings - Accounts - Plus sign in the upper right corner, select the "Import Tangem" option)
- Import your primary account (without the card, as the card is destroyed (inaccessible)) the same way, but select "Import existing account" and then "Read Only". Import with just the r-address.
You can switch accounts in XUMM on the home screen using the "Switch" button on the upper right corner. Your back up card account may display it is not activated. That is OK, the back up account doesn't have to be while still being able to sign for your primary (activated) account.
Make sure you have switched to your primary account. While imported Read Only, because the back up card has been added to XUMM, XUMM will recognize this and silently upgrade your Read Only account to a full access account able to sign transactions (using your back up card).
This procedure when not using Tangem cards: