mixing yeasts

Has anyone tried and had any luck with mixing different types of yeasts?

I have not tried but see no reason why you can't do it.  Two things I'd look for are yeasts that give flavors that work together rather than clash and yeast strains that won't floculate with each other and just sink to the bottom.  There may be some article out there if you google it.  Otherwise trial and error might be your only recourse.  Post your results if you find something unusual.

I know some of the bottle conditioned Trappist beers are fermented with one yeast, and then a fresh different strain is added at bottling.  So if you try to culture the yeast in the bottom of the bottle you won't get the same strain they used in their fermentation. Sneaky monks.  But I haven't heard of intentionally mixing strains during fermentation. 

Try it.

Let us know how it came out.

Wyeast offers a few blended strains. I would email them with your questions. I'm sure they could help.
I would imagine that White Labs would also help, it's just that Wyeast is local for me, that's why I use them.