is it possible to start an account on this instance that can be followed via either Mastodon or Pixelfed
I just looked up your Lemmy profile in my Mastodon app and it looks like I can follow you at least. There’s no visible post history though, so that might be a blank?
and vice versa
I’m not aware of any user follow feature in Lemmy? It’s all centred around community subscriptions AFAIK. That doesn’t mean it isn’t technically possible — it’s the same protocol as Mastodon etc — but if there is no UI option to follow each other, the question is sort of moot.