How can I interpolate between two FFTs?
This is the solution for a linear interpolation between two frequencies as a "weighted average". You would just need to decide what your weighting actually is in terms of magnitude or dB scale for the vertical axis.  X is the interpolated frequency, weighted toward the stronger signal, and Y is the linear interpolation at the new frequency location of the signal strenths a and b.



Here is a simple example demonstrating the results that would be achieved with the formulas given: