You can’t connect two already existing accounts in a multi-entity set-up. To use a two-entity set-up, you’ll need to create a second account using the same email address you are already using in the other organization.
We recommend comparing both accounts and deciding based on the more important physical cards. With the help of an admin, you can transfer all virtual cards before deleting your account and move them back as soon as the multi-entity set-up is on track.