When I do a 2 step decoction I start with the mash in 2 separate MLTs, my direct fire MLT with 40% of the grist (1.5qts per pound), and my Cooler MLT with the remaining 60% (1.5qts per pound).
Dough in the cooler to arrive at your sacch rest temp. Heat the direct fire to 125 and rest for 10 minutes, then bring it to a boil for 10 minutes (or whatever the recipe schedule calls for). Then recombine the grist into the cooler to bring the entire mash up to the final rest temp.
Something is always fermenting....
"It's Bahl Hornin'"
Kegged: Bourbon Barrel Imperial Stout
On Deck: German Lager