Minoxidil is dose-dependent, so you can't expect to be able to cut the daily dose in half without taking a hit in performance.
Having said that, however, I should also point out that when you use 1 mL of a 5% minoxidil solution twice a day, you're already well into the area of "diminishing returns", meaning that if you switch to only one dose per day, the beneficial effect probably won't drop by HALF like you might expect, but by a smaller amount. So it seems to me to be a reasonable compromise to go to one application per day.