Would it make sense to consider asking Tokodon to support connecting with Lemmy servers, or is there too big a difference between the APIs, requiring a separate application?
Technically if ActivityPub instances served as an actual SSO, you’d be able to connect to a Lemmy instance via a Mastodon client, but that’s not how this works.
ActivityPub is all about activity, i.e status updates, messages, etc. It’s up to each instance to verify and maintain the user experience.
If someone were to make a client that supported logging into any instance without the need for the Mastodon API, then you’d still not have a SSO, because then you’d just be connecting to a different instance using a client.
True SSO means being able to create new accounts on a platforms using the identity of another platform, a’la Google, Apple, GitHub, etc. That would require implementation on both backends.
Then there are platforms with nomadic accounts, if they still exist.
Point is there is really no standardised SSO for the fediverse, at least not yet.
You can yes, but you have to decide to either be stuck with Mastodon UX or Lemmy UX. They’re both different and I like to switch UX whenever I please, and currently I need 2 accounts to do so.