I have wondered if people with multiple personalities ever communicate with their other personalities. And how.

I obviously can't speak for Ember or Flame, but for us, our headspace is like an open forum when can share thoughts and feelings freely. Though we're also aware that there are many in which each alter/headmate/personality is kept separate, generally to hide trauma from a host who isn't prepared to deal with what's going on in a given moment. I said all that, but please keep in mind that while I do have my own personal experience to draw from, I'm not an expert in the field of plurality just 8.5 (1 host, 7 additional alters, one alter is also a median system with 2 facets, really the headmate count is 9 counting the host, but our median system don't want to classify themself as two people, rather 1.5) people who've spent a good amount of time figuring themselves out but always happy to answer questions!