Dante,
I agree with you completely on running a single box in stereo. The stereo ambient speaker frequencies will cancel each other out if they are in phase with each other and make it harder for guitar to cut through in the mix. That was something I first learned about when I got my G-K 250ML.
I don't have a split stack setup (using emulation into studio monitors), but once had a Roland Jazz Chorus, so at least some experiences.
The Roland chorus was build to have no phasing-out problems, and produced a lovely ambient yet still localized effect.
I can see this phasing issue with a split-stack with i.e. the MP-2, which isn't really true stereo, but rather a mono chorus going to one side, and the other side simply tapping the mono chorus through a 180 deg phase reverse.
While this does generate the lovely effect we all know, even with spaced-apart speakers, the out-phasing gets horrible when physically moving across the sound scape.
My Digitech TSR-24S generates proper true stereo effects, including chorus, flange and multipole chorus.
I'm getting tired of the studio monitor setup, starting to desire a split-stack setup.
If so, I'll simply convert my MP-2 chorus to TrueMono
purely to thicken the plot, so to speak, and use the TSR mono-in stereo-return in the MP-2 loop for generating true stereo effects.