You can use Mastodon to interact with Lemmy content and vice versa, but generally speaking the user experience isn’t good. Lots of manually typing URLs and trying to figure out what you’re looking at when you get there.

In theory you could host a Lemmy and Mastodon server under the same domain (using subdomains, e.g. lemmy.feddit.uk and mastodon.feddit.uk), but they’d be different servers in most ways that matter. I presume they would maintain separate user account databases (without some concerted hacking).