Is it possible to have more than one primary partner?
Yes. "Primary" does not have to mean "the one person you're committed to above all others." It could also mean "someone you're deeply committed to, see a future with, and will make sacrifices for."
A lot of people think "primary/secondary" stuff is about who ultimately has the 'final say' - like if Hermeneutic is your primary and Metaphysic is your secondary, Hermeneutic can demand that you break up with Metaphysic, but not vice versa. Or if Metaphysic wants to move for his career and Hermeneutic says no, you won't move, but you'd move for Hermeneutic.
That's not what it means, or has to mean. It doesn't have to set up partners as oppositions or placing one 'above' another. "Primary" marks a certain level of commitment, willingness to sacrifice, and place in your life. More than one person can have that place in your life.
But this is just the general case; for some people, wrapping their heads around having multiple primaries is impossible. For others, having any "primary" partner or partners just doesn't make sense. So while it's possible, it might not be possible for every individual.
It's a lot messier when things aren't neatly hierarchical. If conflict arises, you don't have someone to 'default' to. You don't have an external structure to dictate your choices. You need to be flexible, dynamic, intentional, and present. If Hermeneutic wants to move, or Metaphysic wants to become monogamous, you have to decide what's best for you, you have to talk things out, you have to think through all the risks and possibilities. And you have to be accountable to the choices you make, rather than throwing your hands up and going "what can I do? He's the primary!"
If that risk doesn't sound like something you can handle; if the tidiness of hierarchical polyamory feels safer and healthier for you, then maybe having multiple primaries isn't right for you. And that's okay! Something being an option doesn't mean you're obligated to do it. But if it's something you want, and you're just worried that it's "not possible," relax! Do what's best for you and your relationships, and don't worry about how other people frame it.