I havent mixed during fermentation. I did an Arrogant Bastard clone a few years back; 4 gallons with 1056, 1 with some belgian yeast, and mixed them right before bottling. Interesting outcome. I fully intend to brew a batch this year and do a double fermentation; start with US-05 or something, and after a week or 2 reintroduce sugar in the form of a simple syrup and add a different yeast, maybe a 1056 or belgian yeast. I'm curious how yours is going to turn out with all 3 of them going at the same time.