Welcome, @Jason_Barmer,
The short answer, based on what you’ve written:
Upgrade to a two-seat Business plan.
Your user with a different email domain will be free and will be a “Guest,” whereas the other two of you will be “Members.” A Guest can do all common things, but not create new custom fields, create new teams, etc., but can set the value of custom fields; it’s usually not an impediment.
A Division (What I learned about divisions 😅) won’t work for you because it will create silos and the three of you won’t be able to collaborate easily as you’d expect.
More information is at How to use Asana, Asana tutorials, onboarding tips, and team use cases | Product guide • Asana Product Guide if you look up Organizations, Members, Guests, Divisions, etc.
Each of the things you ask about is not simple and beyond a full explanation here, but based on what you said, I’ve made the simple recommendation that I believe holds in your case.
For the full answer, contact sales@asana.com.
Thanks,
Larry