Jeff Beck ran a dual-amp arrangement like what you used for a period, with a lower-wattage Fender, like a Princeton or Champ, in conjunction with a Twin or similar. As I understood it, the intent was similar to yours: push the mids hard into breakup, via the smaller amp, while maintaining cleaner low end via the larger one. As someone pointed out, this is really more of a blended dual-amp arrangement, rather than anything one might call "stereo".
We'll get to the sonic possibilities in a moment, but there are a number of caveats top consider with respect to going stereo.
- Obviously the weight, and cost, of extra gear.
- The frequent impossibility of either situating oneself, or audience-members in "the sweet spot", where the luscious stereo can be heard.
- The tendency for some forms of stereo pedals to cancel out "in air".
- The extra stage space required.
- Bandmates each hearing something different.
- The distraction that can result from two very different sounds coming from two different locations. As I've likely noted here on several occasions, the only time I've ever stepped up to a mic and completely blanked on lyrics was a time when I had a slow Leslie to my left and a separate amp with a slow tremolo on my right.
I have a couple of smaller amps that allow me to play with stereo in a convenient form in my home. I also have a cheap Fender Sidekick SK20 Chorus, with a pair of separately powered 8" speakers that functions as a poor-man's JC120, providing the wet signal on one channel and the dry on the other. Of course, with a cab sized for two 8" speakers, the wet/dry separation requires situating oneself rather close to find the sweet spot. One good feature of the amp, however, is that it has a stereo effect loop, so I can do things like insert the wet part of a phase-shifter into one channel, and leave the other one dry, or even a pair of tremolos or phase shifters. Sounds marvelous, although limited in terms of where the sweet spot is for really enjoying it.
Some stereo effects are too extreme. The Ibanez Flying Pan was a shortlived unit that combined a simple 4-stage phase-shifter with an autopan, and separate sweep rates for each. Fascinating "psych-a-duci" idea on paper, but you couldn't leave the thing on for more than 30 seconds before thinking "Okay, that's enough. I'm getting nauseated." The contrast and degree of activity between the two outputs was just too distracting. Although, if one used a single effect at a time, and only one output at a time, it was serviceable as a phaser or tremolo.
When it comes to stereo modulation effects, I think quadrature LFOs are underutilized. A quadrature LFO provides outputs that are only 90 degrees out of phase, rather than complete inverses of each other. More complex ones will give you LFO outputs of 90, 180, and 270-degree lag. Here's a demo of various phase manipulations using a software-generate quadrature function:
Dan and Mick, over at TPS had some segments on three-amp wet/dry/wet arrangements, that sound pretty righteous.