2 styles of yeast to pitch for 1 batch of whisky

[Fallsworth]

Im curious if anyone has done an ale yeast to start then shortly after pitch m-strain distillers yeast to finish off on a 4 day fermentation on grain.

[SpiritedConsultant]

Something similar used to be common in the scotch whisky industry. Distilleries would supplement their distiller's yeast with spent yeast from nearby breweries, mainly to save money (at least that's how it probably started). The spent brewers yeast wouldn't last too long, but it would contribute some flavor and EtOH and then undergo autolysis. LAB is naturally present in most whisky washes, and can metabolize some of the autolysis byproducts, creating novel flavors. I can did up some research out of Japan on this. Send me a PM if you are interested.

When I make bourbon, I often use a combination of low attenuating, low abv tolerance yeast strain with a high/high strain, for similar reasons. Generally pitch a lot more of the high than the low.

[adamOVD]

Some brewers and cider makers will pitch champagne yeast towards the end of the ferment to dry out the beer or for super high alcohol content. I've done something similar with rum to get a better yield. One yeast for flavor the other for good attenuation.

[Silk City Distillers]

Yes, I love to mixed ferment.  Generally though, I'm doing it because I'm pitching an esoteric slow fermenting yeast first - Torulaspora, Metschnikowia, - or bacteria, etc.  And giving it a solid 24-36 hours to build character before pitching the heavy hitter.

Why the desire to co-pitch?  Are you assuming attenuation is going to be a problem?  At reasonable starting gravities, it's not problematic.

I use a number of ale yeasts, none of which have had any issues fermenting down to 1.000sg.  S-04, US-05, Nottingham, ECY10, etc.  Chalk it up to slightly warmer fermentation temperatures (high 70s), high nutrient (backset), agitation in fermenters to reduce early floc., and mashing with little to no residual dextrins/unfermentables (glucoamylase).  Nottingham is hands down, one of my favorite whiskey yeasts.

Couple things to keep in mind.

In my experience, high starting gravities are going to make this more challenging, not less, especially if your first pitch yeast is a fast fermenter.  By the 24-36h mark, you've already created enough alcohol to create a stressful environment for the new pitch, at that point there may be enough of the Yeast 1 biomass to outcompete Yeast 2.  Reduce the timeframe, and you don't yield nearly the same character from Yeast 1.

To emphasize character of Yeast 1 - consider underpitching at 1/2 typical cell counts - this is to emphasize Yeast 1 character and to slow down Yeast 1 somewhat (yes, it's somewhat counterintuitive).  Underpitching ale yeasts and fermenting them warmer than recommended temperature is a great way to emphasize ale yeast ester contribution.

Killer Factor Positive and Killer Factor Sensitive yeasts.  If Yeast 1 is sensitive, and Yeast 2 produces killer factor - this may be counterproductive.  Pitching a sensitive ale yeast, and then a fast fermenting KF+ yeast a few hours later - you might be wasting money.

Careful with the high-phenolic POF+ yeasts - those phenols come through loud and clear, screaming all the way through hearts.  Might be your thing if you like a "peaty"  phenol character.

Really fast ferments (4 days) is not going to yield tremendous yeast flavor contribution, so it may be moot.  Fast ferments are ideal if the end goal is a cleaner spirit with higher yields, but longer ferments really begin to emphasize character - I think this is especially so with the ale yeasts.  Let 'er riiiide.

Maturation duration is a killer of unique yeast flavor contribution.  The beautiful fruit and florals of ale yeasts do not stand the test of time.  You can have an amazing new make, by the 1 year mark it's already lost considerable character, by the 2 year mark, it's almost gone.  It becomes a very, very subtle difference.  I always wondered why the big distillers said that yeast didn't make a difference.  It's not that, it does make a huge difference in new make ... but at 4 years?  8 years?  12 years?  

Concur with @adamOVD on the Champagne as Yeast 2 - EC1118 is very commonly used to rescue stuck fermentations, as it can withstand the stress of being pitched into a higher alcohol fermentation.  FYI - It's killer factor positive.

[Silk City Distillers]

Experiment I've been meaning to try - lay down a few barrels of a heady-cut ale yeast whiskey for longer term aging - to be used as blending stock with a more traditional cut/fermented whiskey.  The more liberal heads cut to try to counteract the loss of ester-character in longer-aged spirit.  Obviously you wouldn't go to the bottle straight with something like this, but it might be an interesting way to reintroduce more ale yeast-character into the final product.

[captnKB]

I do mixed ferment ferment for all of my bourbon as there are some ale yeasts that bring amazing complexity to bourbon, that does not seem to come from strictly distillers yeast

[adamOVD]

I use bakers yeast to finish my mixed ferment rum. Even if I only get 1 extra bottle of yield, it is worth it foe a 4$ brick of yeast. I haven't noticed much of a difference in flavor from doing a mixed ferment as opposed to ale yeast only. It would be hard to to do it all in 4 days as the OP wants to though.