From what I understand, and someone correct me if I’m wrong, you don’t need multiple accounts (unless one instance has blacklisted another). You can subscribe to a community on a different instance and be able to comment and post without creating an account on the second instance.
For example, on kbin’s search page you can search for programming@programming.dev and subscribe. programming.dev is a completely separate instance running Lemmy with its own communities. Then you can see content from there on your subscribed page.
Aglets. Learned that from Phineas and Ferb.