The enzymes beta-amylase and alpha-amylase have ideal ranges. Doesn't mean they will not work they just take longer if a little too high or low.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4460087/#!po=49.0826
Basically what it says is that our brewing enzymes will still function until pH induced denaturing, which happens around pH 2.0 Though they lose a lot of functionality below 4.0 pH.
But for your application I would mash normally until saccrafication is done then add your acidulated malts at the end. Sparge once you've reached your desired pH.