@Reshirams_Rad_Slam@nostupidquestions this would be possible if someone made a platform that implemented the same APIs for both mastodon and lemmy, but would fail if there was a platform they didn't implement say Loops and you tried to log into their app. A few fedi microblogging platforms like Plemora already implement Mastodon's api to allow using the Masto apps with their instances for free.
Federated identity is something being worked on, but will be likely be seperate to AP itself.