Firstly, when you use these terms "primary" and "secondary", what exactly do you mean by them? What I would mean is level of practical entanglement - shared finances, kids, a home - that kind of thing. If I don't have those things with anyone, I don't use them at all. I might say that I am more compatible for a particular type of relationship with one partner than the other but neither partner is primary or secondary because we don't share the things that entangle us in that way.
Some people use these terms to define which relationship, which person, they can love the most, which one is the priority. And that means that any other relationships will involve less emotional availability, less love, so as not to be seen to compete or threaten the primary relationship.
The reason this is important for you is because that means that your relationship with her will be promoted to the primary spot when it's empty and thrust back when it isn't. The thing is, that won't take your feelings into consideration. You will be expected to love completely when the spot is vacant, and love and be loved less when it isn't. Is that what you want?