I use 2 different yeast strains on occasion but not for the results your looking for. I'll use a British strain and the add a high gravity strain latter in the ferment, to get the flavor I'm looking for and the abv.
For what your proposing I haven't tried it but if I where to guess one is going to take over pretty fast. I would try to figure out the stronger strain and add that a couple of days after the the weaker strain is introduced. I Really don't know but that's what I would try.