I’m scrolling through old posts and figured I’d give an answer to this in case someone else comes across it and is wondering. Regardless of if the instance is a third party instance that’s federated with two instances that are defederated (ex: beehaw and lemmy.world), neither can see the other user’s posts or comments regardless of where they are.
I was scrolling through !opensource earlier and there is a popular post that I can see from another account, but I can not see it on my beehaw account, because the poster is a lemmy.world user.