People can add 2 bitcoin addresses when they sign a message and that message will be use as ownership evidence for both 2 bitcoin addresses.
this sounds like an entirely different thing and it is possible to do. 2 different people can create two different keys and then share their public keys with each other to add and then come up with an aggregated signature which would be a single sig but coming from both of them. that is simplified version of what Schnorr signatures do.