Take a look at the MBTI. Only a crappy test (though there are many) would operate on trying to figure out if you were F vs T, and then S vs N, etc.. in such a manner. Any test that's worth anything is going to ask questions to determine how much you use processes like Fe or Si. It's going to look for the distinct qualities of their interaction.
As such, the yet unwritten test for this system would likely not try to determine your type in each variable in an isolated fashion. The exact mechanics I do not yet know (obviousy, or I'd have a test), there are a few choices on how to do it. But whatever way it's done, it would be feasible to account for the interaction of two traits. Because the behavior trait is more about a default strategy for how to situate yourself, and the instincts are more about what you particularly want or don't want. There is a distinction to each of the 9 possible combinations of behavior and motive.
The biggest issue for me in writing a test would actually just be the sheer complexity of accounting for all three variables. That is, also including the affective/mood variable.
I hope that's somewhat helpful.
Let me say that the type One descriptions usually do a very good job of describing me. However, things really fell apart for me at the wings and the lines of intergration and disintegration. This lead me to further investigation of how they worked, and I found something I considered dubious at best. So in other words, this project comes about as a result of the incredible inconsistency with which the Enneagram works for people.