I had a friend try it, and as Evan says, one yeast kicked the other's ass, so it was indistinguishable from a single-strain batch. Maybe you could add the stronger yeast after a day or so? I guess you'd need a decent sized starter to help it overcome the superior weight of numbers of the lesser yeast. Whether you'd get a pleasant combination of the two, or whether it'd just be an odd beer, I don't know.
Actually, a better alternative would be to split the batch entirely and ferment each half with one yeast, and then blend them at bottling time. That'd allow you greater control over the outcome, as each yeast would be able to express its qualities in full.
Bottle conditioning: Pliny the Elder clone; Tramp's Overcoat Barley Wine
Next up: Vanilla Porter