MOTM

>Anyone use multiple frequency shifters?

Yes.

>I'm just curious if you could do something like
>
>OSC1 -> FREQ1 -> VCF -> FREQ2
>
>and somehow get the cv where if you shifted FREQ1 up by 5hz, FREQ2 
>would go down by 5Hz.   

Here's something I produced with such a setup:
http://jhaible.heim.at/matrix_fx/jh_fs_updown_demo.wav
(7 MB *.wav file. A *.mp3 didn't capture the sound.)

Background information:
http://jhaible.heim.at/matrix_fx/jh_matrix_fx.html
(Scroll down to "SSB Modulators".)

JH.