If lemmy created a community the same name as the username when someone signs up. You could follow a lemmy user and then when your on mastodon (once we’re federated with mastodon) you could see their self posts in their community as a mastodon post and vice versa.
I do wish I can go to other lemmy instances with the same account or post there as well
That’s not how federation works, the whole point is you don’t have to go to the other site to make posts, you can make them and view them from the one you’re on. And this currently works with lemmy.