If you are using spironolactone 2%.. apply the spironolactone first, let it sit 10-15 mins, then apply the minoxidil.
If you are using spironolactone 5%.. apply the minoxidil first, let it sit 15-20 mins, then apply the spironolactone. That is what Dr Lee says, but personally, I let the minoxidil sit a LOT longer than 20 mins before applying the 5%. I usually let it sit an hour if I can.