Originally Posted by Tenspeed
The consensus seems to be that one yeast strain will come to dominate
It's funny, that seems to be what most people say but in the book Yeast by Jamil and Chris White they argue differently. It's actually page 56 (I felt the need to reference this and make sure I wasn't remembering incorrectly) and they say that in the wild many yeasts compete against one another and some have kill factors and will dominate others but brewer's yeast doesn't work this way. Brewer's yeast though is different, perhaps because it's been controlled in a lab for so long that they will not compete against one another.
They also talk about pitching yeasts at different points in the fermentation process. For example, if you wanted Nottingham's flavor you would pitch that first and wait two to three day and then pitch the US-05 to chew through the more complex sugars and get the attenuation that it is known for. Adding both at once will add characteristics from each yeast and create a more complex taste profile.
This is all the book speaking, I have never actually mixed yeast but these guys seem to know what they are talking about.