Anyone that is interested in discussions on either defederated community, will create an account for both.
And that is the reason why reddit is still growing. If you are required to make multiple accounts just to engage with the communities you want to engage with, Lemmy is no better than separate forums. And those all got overshadowed by reddit for a reason.
This is an issue on Lemmy as well, one that very few so far seem to have wisened up to. If you host a server and federate it, your server will pull, store and display the content your users are watching for all to see. So you could be on the hook for distributing illegal material if you don't preemptively defederate from instances potentially hosting content that is illegal for you to possess.