Below is a list I've compiled over time of pedals I know of with stereo inputs. Yes, they have stereo outputs, too, but that's a given.
Feel free to post more, and I'll add them to the list!
--------------------
REVERB PEDALS WITH STEREO INPUTS
Boss RV-5 Reverb
EBS DynaVerb
Line 6 Verbzilla
DELAY PEDALS WITH STEREO INPUTS
Boss DD-6 Digital Delay
Boss RE-20 Space Echo (Reissue)
EHX Stereo Memory Man with Hazarai
Line 6 Echo Park
Line 6 DL4 Delay Modeler
Line 6 MM4 Modulation Modeler
VOLUME PEDALS WITH STEREO INPUTS
Visual Sound Visual Volume Pedal