Not equal polyamory, which can almost never happen naturally. You're never going to be everything to one other person, even if you're Destined to be Soulmates. And they're never going to be everything to You.
It's totally valid to be someone's side candy for a member of another couple, or live in a polyamorous household of five furries, everyone with their own set of boundaries and needs.
The secret, the start and finish, is Talking about your needs, and what you have to Bring.