It's not a perfect fit, so if you want to use both Lemmy and Mastodon I strongly advice you to have an account in each.

That said, you can follow Lemmy users or communities on Mastodon, likes made on Mastodon to Lemmy posts equate to upvotes here. If you reply to a comment sent from Mastodon, the Mastodon user will see it. But to fully enjoy both, you need 2 accounts.

I started with a lemmy.world account and i was fine with that. Then a couple of weeks ago lemmy.world was down for a day so i made a Mastodon account. It was functional in that i could see the posts about the outage but i don't care for the layout so I made a lemm.ee account as a backup. They all show me the same stuff.

I think most things are the same just presented in a different format. However, for those using Lemmy apps, they don’t seem to work with kbin, so there are differences.

My understanding is that any service that uses ActivityPub can see everything from every other service, unless the developers choose not to show it. As for whether Lemmy's developers choose to show stuff from Mastodon, I'm not really sure.

They all should be able to speak together, it's more a question of how each interprets it. I havn't tested personally but some reports of following lemmy from mastodon results in a firehose of individual comments for a given topic being received.

Think that in the background Lemmy and Mastodon uses the same protocol, which means that you can consume content from one to another.

What changes is the ui, where Lemmy organizes the content in a reddit-like experience and Mastodon is closer to Twitter