Open fermentation can yield some amazing flavors, but requires the brewer to pay special attention to temperature, cleanliness, and fermenter shape. Some professional breweries cool their wort in vessels called coolships. Coolships
Brewers often add adjuncts right into the main batch of beer before or during fermentation with the hopes that the characteristics of the beer will play nicely with the additive. But Michael Tonsmeire explains that fermenting the two separately may sometimes be your best approach.