Well, I don't really understand the how the Buttkickers and Bass Shakers work and maybe you can't measure them, however, there are people using the BFD to equalize them for whatever reasons. As earlier stated, I don't think I'd buy an extra BFD to do this, but if you have an extra channel you might experiment with it.
Here is the typical response of a bass shaker:
It may be well enough to just turn up the volume of the Buttkicker amp. There has also been discussion about using the BFD to create a lower cutoff frequency.