I have separate accounts for Lemmy and Pixelfed/Mastodon. Their content is so different, that it's hard to find a unified way of interacting with it (following accounts vs. subscribing to and interacting within communities).
If you still prefer to combine both, you could make an account on kbin.social. With that you can follow both Mastodon and Lemmy content and get a "unified" UI. I cannot say how well it works though, since I haven't tried it myself.