I would add both yeasts at the beggining so you get some influence from both. If you add one after 90% of the fermentation is over the second strain wont do much at all. Belgians are probably the best beers to blend yeast with so give it a shot and let us know what happens. I know when I talked to white labs about blending they told me you could get about 3 generations before one strain started to dominate. I imagine that if you blend a highly flocculent yeast with a poor flocculator you would have to be careful when harvesting to maintain the blend.
"You are what you is." FZ