As a technicality, yes. Consider

• https://mastodon.social/@Gargron/107296292152663942
• https://lemmy.ml/comment/92515

The same thread, viewed in mastodon and lemmy!

In practice, it doesn’t work this way. The communities rarely intersect. Future questions are better for !lemmy_support@lemmy.ml, but welcome to lemmy

Yes, all the services share the same underlying protocol and share content.
If you go to your frontpage and switch from “local” to “all” you’ll see posts from many different sites and services.

You already posted this from sh.itjust.works to lemmy.ml, so you’re already doing it.