Keep it simple. Half the mash recipe and prep 2 batches. Mash the first in full volume treated water (liquor). This provides sufficient liquor for the second mash, which doesn't need any more water treatment. Due to all the first wort molecules from the first mash, enzymes and substrates collide at a lower rate and lower the efficiency of the second mash. You'll need to determine this empirically for your own equipment, ingredients and procedure. I wouldn't try to compensate until you have some idea based on mashing your recipe as is. You can always add some sugar later in the boil or in the FV. It's not going to be detectable in the end product.
