Well think about it with this crude kind of inaccurate analogy.
You have a windows laptop. Your friend has a windows laptop. When you're logged in to your laptop you can send your friend email. And see his emails to you.
But just because your laptop is windows and his laptop is windows doesn't mean your windows log-in would work on his right? Lemmy works more like that. Reddit is kind of like one large windows laptop and everyone gets their own keyboard. Your log in works no matter which keyboard you use.
You may notice that Lemmy communities have the @ symbol like an email. So tech@lemmy.world is different from tech@lemmy.ml (just like how robert@yahoo.com is not the same account as robert@gmail.com). They MAY be made by the same Robert but there's no guarantee.
You really just need one account. So in the communities tab from your instance (Lemmy.world) you can search for the community on the other instance (Lemmy.ml) for example tech@lemmy.ml.
Your account let's you post and comment on @lemmy.ml posts