Do you both even want the same kind of open model relationship?
That would be the first step to me. Otherwise you could end up struggling with apples/oranges. If you don't really want to go there don't
-- there's nothing wrong with (monoamorous AND monogamous) shape relationships.
Just some questions you could think about together:
How do you plan to deal in poly-hell feelings? Like Jealousy? Or more jealousy? Have you decided? Are you both already good at articulating and processing emotions? Or do you have intrapersonal or interpersonal skills to learn before going there? (Being willing doesn't automatically mean being able yet.)
How do you plan to handle conflict resolution? Have you decided?
What about sex health/sex hygiene? Are you both on the same page about that? Including what to do about accidental pregnancy?
How do you plan to Open and avoid pitfalls? Do you agree on that? Or is it feeling "too rushed" to you?
If things don't fly, how would you like to break up? Would you like to be good exes only? Or good exes and friends?
Some links to help you: