To add on
The format of Lemmy/Reddit is forums with threads, where you subscribe to a particular community and make posts / comments for that topic
The format of Mastodon/Twitter is microblogging, where you follow individual accounts and read their posts.
They both CAN communicate with each other, but the format won't always make sense. For example, you can follow a Lemmy community on Mastodon, but it will pretend that the community is a person, which makes it a weird experience. What DOES make sense is using your mastodon account to comment on Lemmy threads when you want to do that.
Ultimately I separate it like this
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account with my real name on Mastodon, to follow friends/family/public figures
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anonymous account for Lemmy content, so I can discuss freely