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I love this concept. Do you feel you have better control over the flavor balance using two levains, or is it just a time saver?
So far (in my experience) I think the answer to the question of control is - yes, to a degree. Working with two levains certainly makes things easier. In the case of pain au levain, the desired acidity is fairly low anyway but I can see the same principle being applied to other breads.
I've previously used it in rye bread - where a high hydration (133-150%) rye starter is used together with a stiffer rye sour. This had two advantages - firstly adding instant yeast became unnecessary, and secondly, the flavour! :) I guess it's a bit like different stages of the detmolder process running concurrently.
With all that said, the hydration and fermentation of the final dough will still have a large effect on the overall texture and flavour. The ratio of preferment to final flour becomes an important limiting factor.