Do you mean two layers on a substrate, which has refractive index n0? What are the assumptions in your computer generated data? Constant refractive index for both layers? Lastly, have you tried this, and got it to work, for a one layer system?
Intuitively, it seems to me that what you are trying to do is not possible. The fringe spacing depends on the optical pathlength, which depends on the refactive index and the layer thickness. But with only a single spectrum you have no way to separate the contributions from the refractive index and the thickness.