The progress spinner is a known issue that has been brought up to the Lemmy devs; if you try to log on where an account doesn’t exist, or if you try to create an account with a duplicate email or username, Lemmy doesn’t give any sort of error, or even stop, it just sits there.

@[email protected] covered the rest of your question; you can create an account on both communities, but it’s really not necessary because of the federation. Hopefully the ActivityPub and fediverse developers make some QoL improvements to this in the future because it is a little clunky coming from something that’s been in development for over a decade.