using more than one yeast for fermentation at once

[guinness310]

cheers mates. 

My co brewer asked me a good question today, he asked if one could use more than one yeast (we use the WLP liquid viles) in the primary. HE wanted to choose two different ones that maybe had different characteristics like WLP051 and WLP057 for example. The yeast is for a 5 gallon batch and we wouldn't make a yeast starter, we would add 2 viles instead. but i didnt have the answer. would different yeast's clash? Thank you for your answers. 

-cheers.

[Tim Roettiger]

I once mixed a lager and an ale yeast in the primary fermenter. When bottling, the beer had a definite lager character. However, after three months in the bottle the ale character had definitely come through. In the end the ale definitely was dominant.