Waves Doubler works for me when emulating that classic dual Eventide 910 trick. Also Digidesigns DPP-1, Pitch Blender, Eventide H949
plugins. I'm sure there are more.
A short 'How to Guide'
by Charles Dye and how he approaches it with the Digi DPP-1.
Dual mono H910s on a stereo Aux return would probably do the trick.
If you use something like the Waves Doubler or DPP-1 you could insert an LPF at 16k or so to take off some top end. And maybe even a low fi plugin to reduce bit depth etc. Probably help to emulate 910's which had less bandwidth. Or you could output these thru a couple of outboard SPX-90s in bypass mode to 'dumb down' the sonic quality.
I use this effect often for widening mono signals and thickening stereo ones.