If you have a dual channel amp with multiple inputs where the two channels are out of phase with each other (like a Vox AC30), you can use a compressor such as the MXR Dynacomp where the input is out of phase with the output, and use a patch lead across two unused amp channel input sockets. This works. Richard Thompson nailed this technique. I built a compressor from a kit that emulates this approach by allowing you to mix in the wet with the dry signal actually inside the pedal. I asked the company for a circuit diagram but they wouldn't provide me with one. I was sore tempted to "reverse engineer" it from the photocopy of the circuit board but never got around to it. Google "Paranormal" and / or "Dinosaural" and see what comes up. Because of their reticence, I wouldn't dream of posting an image of it on a forum, but I don't mind PM-ing a .pdf of what I have (if I can find it quickly enough that is!) because you might have lost your instructions and needed to fix some loose hook up wires... It's a neat bit of kit but good luck trying to track one down, and mine's going with me to the grave!
edit: Try googling "Slide Rig" or maybe "Sliderig" I'm not sure whether this will do what you want, but it contains two compressors and may have facilities that meet your needs. It's also very well thought of by those "in the know" trying to nail Lowell George's elusive t0n3.
As a follow up to my last post, after a long wait, I purchased a "SlideRig" (original full sized job, now discontinued in favour of the latest more pedal board friendly size version.) I can confirm that if the closing licks on Little Feat's "Roll 'Em Easy" on Dixie Chicken float your boat, then the SlideRig will induce a grin that will crack your face from ear to ear. I have absolutely no connection with Origin Effects etc, and offer apologies if this falls foul of any rule about linking outside The Forum.