The Redshift (BoE Whetstone) is to the Small Stone what the Fuzz Factory is to the Fuzz Face, only perhaps more-so.
I've worked out all the mods that BoE added to the Small Stone and subbed them into a build plan for the Phase 90.
While both are 4-stage phasers, the Small Stone is OTA-based, Phase 90 utilises bog-standard op-amps with matched JFETs — so they do sound different and respond differently.
Since you're doing two of them, perhaps it'd be easier to go the OTA route. Then again, if you find a perfectly-matched set of 8 JFETs... you'd still run into some other component-tolerance deviations, but negligible?
Consider an altogether different type?
How does a phaser pedal work and which phase shift type is best for guitar? JFET vs OTA vs Optical phaser comparison.
function-fx.com
Oh, and for anyone that cares to do so, here's the mods needed for making a Small Stone a Whetstone (or a Phase 90 a Whutstoned):
GREEN CHECK

FOR EVERY WHETSTONE FEATURE THAT CAN FIT INTO A PHASE 90 (or similar) BUILD — some other notions, too
WHETSTONE CONTROLS:


FEEDBACK (also known as resonance regeneration)


RATE [
BIG]


DEPTH [just the WET SIGNAL]


LEVEL / VOL [COULD ADD A SMALL BOOST, BEW’S IS MOSFET]

➢ RANGE :: NORMAL/ PADS/ RING/ FIX (the latter may have to be a toggle, two togs vs 1-rototone)


𐄞 STAGES II/IV WIRING THIS TO A POT: DEPTH’s (V)R.


𐄛 VIBRATO/PHASED ie DRY-KILL switch [HAVE THIS RE-ROUTE DRY TO “DRY OUT-JACK”]


𐄞 ASYMMETRIC/SYMMETRIC :: [AFAIK, THIS IS “VIBE” CAP VALUES—could go all separate SWs]


𐄞 LPF ON/OFF


𐄛

SHALLOW/WIDE [MAKING THIS A POT “SWEEP”]


𐄡 STOMP ON/OFF
Haven't had time to implement any of that yet.