This is my first post so I hope I haven't asked anything silly. Can I mix a high alcohol (attenuation?) yeast (for low final sugar and specific gravity) and a yeast that produces more "fruity esters" (like a ale yeast) together into my beer wort. I want to make a dry but more full flavored beer and I wondered if that might help do it.
This can work very nicely from my experience, I often use S-04 and US-05. The S-04 gives esters as you suspect and the US-05 gives a higher attenuation and a thinner body.
The majority of esters are generated during the first 3 days of fermentation and the major difference of a high attenuation yeast will not really be apparent until the later than this when the ABV get >5% or most of the mono/di-saccharides have been consumed.
It has worked well for me exactly as you suspected it might, from 25l batches up to 800l batches. I use other combinations of yeasts to get other flavour profiles. The highest number of yeasts I have used in a single brew is 5, 2 standard brewing yeasts, 3 Brettanomyces and we had some lacto in there as well.
Enjoy layering the yeast flavours!
You can do it, but there's no way to control which will dominate so it's a crapshoot with hard to predict results. I prefer to go another way... split the batch and ferment each half with a different yeast. Then blend post fermentation.
Others have apparently used multiple yeasts without any difficulty and have achieved the desired affect, so my answer below is apparently wrong. I'm leaving it here because it offers some alternatives which I have had success with. Both blending and fortifying beer can produce drafts which are strong in both flavor and proof.
I don't think that will work as you expect it too. The high alcohol tolerant yeast will quickly drive down the specific gravity to an alcohol level that only they can survive, leaving your ester produces to perish and settle down into the trub.
You can probably get the effect that you want by splitting your wort and giving each yeast exclusive access to half. Then after all fermenting is done, (or better yet after each half has aged and mellowed,) you can blend the two finished beers in different proportions until you find a mix that you like.
To simplify the process, you could always just brew your beer for the flavors you like and then mix in some brandy (or whatever hot liquor you prefer) to give it a stronger kick.
I agree with other answers that it might be difficult to gauge the fermentation rate or metabolic requirements of one strain of yeast against the other. However that would not make the proposed idea of mixing yeasts unusable. It would just make the idea indeterminate until the process was better understood from experience. If one can overcome the learning curve then there may well be a valuable discovery at the end. But one wouldn't know until one has tried it.
I note that other advice is to split the batch into the correct ratios and ferment each separately. That is quite reasonable however the batch fermented with a low attenuation yeast would add sweetness back into the final mix. I accept the mixed ales might further ferment and become less sweet, if so - perfect!
As splitting a batch can be a problem, there is an alternative approach - the successive pitching of the different yeasts. I would suggest first pitching the lower attenuating but higher flavour yeast. Give it unfettered access to ferment the wort. One might even try a sample to see how the taste is progressing. When the yeast has done its job or reached its limit (or whatever test) one can then pitch in the higher attenuating yeast to complete the job of fermenting remaining sugars. One could in effect "time split" and not "bulk split" the fermentation process.
Perhaps the only downside to this approach is that the second pitching of yeast might need some aeration to provide oxygen for the second batch to grow. One could rouse and "aerate the brew" by vigorous stirring. Another option is to grow the yeast to sufficient quantity in a (smaller) separate vessel, aerating as needed over a day or so. When ready, pitch the whole load to complete the job in the anaerobic conditions in the main brew.
How one might approach mixing more than these two "obvious" types of yeast I don't know. One could only do it a few times and see if it yielded useful result. Good luck!