Are you talking about making an electronic circuit? Are you talking about putting it in the mic line? Does the mic need phantom power?
It would be much easier to do this in the mixer (whether physical or in post-prodution mix-down).
You didn't mention whether this was for a live real-time mixed performance, or whether this is for a recording session with separately-tracked microphones? The optimal way of doing this depends a lot on details not disclosed.
Ok. Thanks for responding. More details. For live giging in clubs. Trying to simplify my keyboard rig. Organ patches go to leslie(actually a Pro3). I mic the rotating horn with a SM58 into mixer along with the simulated low rotor, my vocal, and an Accordion for my keyboard monitor. I know. It's messy. I roll all the lows of the rotating horn channel at the mixer but I would like to use a smaller mixer and send the horn and the simulated low rotor to both sides of a stereo channel. Hope that made sense. Thanks