Interesting that you're using Belle Saison as your co-pitch. Are you getting phenolics (clove, pepper, etc,) in your beers using this blend? Also, do you keg or bottle? Belle has a tendency to keep going slowly down to 90+% attenuation after the bulk of fermentation is complete (at least when used on its own). I'd be worried about bottle bombs if I packaged it at 83% attenuation unless I let it sit for a week or two at that gravity first.
I am getting a little bit of Saison phenolics, but not much. It was actually what I was going for in the Pumpkin Ale, thinking it might augment or suggest the nutmeg / cinamon / clove / ginger part. I just barely bottled it so we'll see. Both times I've used this combo, I've let it ferment for 3-4 weeks before bottling, so I feel hopeful about avoiding bottle bombs... *fingers crossed*
I suspect the reasons the AA is only 84% and the saison phenolics only mild, are:
(a) the Belle Saison portion is only about 10%-15% of the yeast blend, so Windsor does most of the work;
(b) the Windsor takes off fast and is done in about 36 hours, while the BS is notoriously slow, and has little left to work on;
(c) I am fermenting on the colder end (66 degrees) of the range for BS, probably suppressing phenolics somewhat;
I just barely bottled the Pumpkin Ale, so it will be a month or so before I know if I was successful in what I was going for, but the gravity sample was super nice.