Hmm... That's an interesting recipe. I had always heard that if you combine 2 yeasts, that the stronger strain will just dominate and the other will basically contribute nothing to the finished product. Judging by the author's comment at the end of the recipe, he says you can notice contributions from both yeasts. Who knew?
As for when to add, I would assume they go simultaneously as well - if not, one is sure to overpower the other.