You could do it where all messages are forwarded to your desktop. It is ok to have every device have a new ID. Bens phone, bens computer. "But what about when a user sees an unfamiliar device send him a dm, I verified bens phone but not bens computer." You can have it have a badge thatsays verified by bens phone.
Not sure if it works like that with fixed public keys. Or at least the initial interaction can be with fixed public keys before switching to signal protocol