This is the right forum for questions.  Sometimes, it's not that active, or the answers don't come back immediately.  I've also seen a lot of interest on Eventide products at The Gear Page, in the Effects, Pedals, Strings and Things section.

There are a few pre-installed presets on the PitchFactor that should give you what you want.  Have you run through all 100 presets yet?  If not, this preset below should get you started:

This preset is set up for an E minor scale; playing thirds above your input notes in both pitch shifters.  If you want to use another scale, make the adjustments with the Key and Scale knobs.  If you want to use harmonies in 5ths or octaves, make those changes in the Pitch A and Pitch B knobs.  I added some very short delays to keep the voices separated, but the Delay A and Delay B settings are optional.

To be honest, I've never tried.  My first thought was that you'd need a slow attack parameter (like in the H9 UltraTap algorithm), plus some octave shifting in the PitchFactor, and tone modification from some other pedals.  Your playing technique will be a huge part of this (thinking like a cellist).

I'm not sure what you have in terms of a pedalboard.  I would think that you'll need a fuzz or compressor, and a volume pedal at a minimum.  As far as doing it alone in the PitchFactor, it might be possible to come close with the Synthonizer algorithm.  If I can manage a reasonable facsimile of a cello in that, I'll post back into this thread.