How can I interpolate between two FFTs? I have two FFTs that represent two impulse responses. I want to interpolate between the two of them to generate a 3rd FFT in the manner shown in the diagram below. This is to be used in real time audio rendering filtering between two FFT filters. 

Linear interpolation between each of the FFT's complex coefficients doesn't work as wanted.

FFT plots of two impulse responses 
(x = frequency/coefficient, y = frequency magnitude)

+                                   +                           
|                                   |                           
|        +                          |                           
|        |                          |                           
|        |                          |                           
|        |                          |                           
|        |                          |                           
|        |                          |                   +       
|        |                          |                   |       
|        |                          |                   |       
+--------+------------------+       +-------------------+-------+    
           Input A                              Input B

FFT plots of interpolation between A and B (Linear, and Wanted)
(x = frequency/coefficient, y = frequency magnitude)

+                                   +                                
|                                   |                                
|                                   |                                
|                                   |                                
|                                   |                                
|                                   |             +                  
|        +                          |             |                  
|        |                          |             |                  
|        |          +               |             |                  
|        |          |               |             |                  
+--------+----------+-------+       +-------------+-------------+    
Linear Interpolation (A+B/2)            Wanted Interpolation


How do I do the interpolation as described in the Wanted Interpolation graph?
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: