You can join both, sure. You'd just independently click the Subscribe button on /c/[email protected] and /c/[email protected]
Because they naturally each have different rules, cultures, members, and such; having the same community on different instances is mostly thought of as a feature, not a bug. I've seen comms be toxic on one server but thriving elsewhere. The hope is that you can organically sub wherever you want to participate or are seeing what you want to see, and weed out bad communities or ones that die out.
There's are GitHub issues around "multicommunities" for every client, but I haven't seen anything up and running. Example issue.