If I recall, Dr. P has stated in the past that he wrote that a long time ago--back when 2% minoxidil was the only minoxidil available--because he was worried that if Prox-N (with all it's penetration enhancers) was applied directly after minoxidil, it would cause too much minoxidil to be absorbed.
Years later, now that we have tested higher strengths of minoxidil, it is of little concern in regards to safety, and Dr. P has repeatedly said that it doesn't matter when you apply them, BUT to avoid possible dillution of either substance, it is best to wait a little bit until one is dry before adding the other.
My advice? Add Prox-N to a well-hydrated scalp, wait 20 minutes, then add minoxidil over top.