Because you can't run multiple accounts at once? Either I'm misunderstanding what you're saying, or you're missing the point of a signal backup.
OP is referring to the GrapheneOS feature, well really the android feature heavily enhanced by GrapheneOS, that allows you to run the OS as another user with their own apps and files. OP is asking for help running signal (Molly) in both profile A and profile B as one account. This is technically possible but realistically infeasible because of the way profile switching works on android. I explained why in a different comment.