We very often see the same username created across many instances

Guilty as charged. I'll say though, there are several legitimate reasons why one might want to do this. I personally use it as a substitute for Reddit's multireddit feature, by grouping community subscriptions across different instances by theme. As long as users use the same username across instances I don't think this practice should be automatically regarded as an attempt to sockpuppet. It that was the goal, the accounts would definitely not be using the same username across all the instances.

Thanks for that input, that's very cool that those relays exist. I have to apologize anyway though because I apparently was pretty confused when I made the original post, because on Lemmy you can't directly follow other Lemmy users at all, whether they're on Mastodon or Lemmy.

Something like what I envisioned might still be workable, but it would take a bit more effort and savvy. It would probably require a Mastodon hashtag bot as well as a dedicated Lemmy community for each tag for the bot to post into. See my edited OP.

When you search on a mastodon instance, some of the hits will be users. You can narrow down the search to show only those by explicitly specifying so. A segment of those users often look weird because they have no name at all and always share the standard lemmy default icon as profile picture. Like this:

Those are (usually) non-Lemmy Fediverse users, most of which are on Mastodon. You can click on them to see the profile and from that you should be able to follow them.

Edit: While it’s true that you can find Mastodon users on Lemmy using the method described here, you can’t actually subscribe directly to another user to get their posts in your feed, whether they’re on Lemmy or Mastodon. This notion of users directly following users is very much more in line with Mastodon’s design philosophy than Lemmy’s, so you’re probably right that you can follow a Lemmy instance from Mastodon but not the other way around. Please excuse my brain fart.

The only way I can see to make anything similar work would be to have hashtag bots like those described by @RxBrad@lemmings.world automatically post into relevant (or separate specialized) Lemmy communities, which admittedly seems like it might be interesting. Whether something like that already exists, though, I have no idea.

I didn't mean federate in the Lemmy/Fediverse sense, but rather in the collation/multireddit sense. In other words, gathering posts from many different sources, in this case instances, and displaying them as a single list. Given the context, calling this "federation" was probably very confusing, and definitely a poor choice of words. I apologize for that.

In Liftoff, this takes the form of an All/Everything view (where "All" means all currently logged in instances) which collates and displays posts from many different Lemmy instances at once. This is the feature I was looking for in other clients.

In this context it probably is, since the federation that is done on the fediverse is something else entirely. "Collate" might be a better word, perhaps.

See my answer to @rowinxavier

This is what I've been doing to log into new instances, but it always seems to just replace the instance I was already logged into with the new one. The UI looks like it's supposed to show a list of instances and the button says "Add account", but the list is always empty, no matter if I've logged into other instances before.

Workaround