Disregard concerns of extracting tannins when decocting, as they do not apply. The boiling and mixing deaerates the mash likely resulting in a PH and density preventing phenols from leaching out of the husks (Noonan, 1996, p. 136). The majority of enzymes are found in the thin mash therefore use the thick mash until you are ready to denature your enzymes, at that point, use the thin mash. Bring your decoctions up to the typical rests of protein, beta, and alpha, holding 15 minutes or so at each stop, before coming to a boil. Boil 15 or 20 minutes and return to main mash. "Mash thickness also affects the fermentability of the wort. A thick mash (less than three-tenths of a gallon of water per pound of malt) induces the greatest extraction" (Noonan, pp. 140, 141). If you are relying on the decoction to bring the main mash to the next rest temperature level, a larger amount of heat energy is transferred in a thick mash per volume compared to a thin mash. "One pound of crushed malt contributes about the same amount of heat to the mash as does one pint of water, yet displaces only as much volume as six fluid ounces of water" (Noonan, p. 137).
Reference:
Noonan, G. J. (1996). New brewing lager beer. Boulder, CO: Brewers Publications.