Minoxidil twice a day is unnecessary, the half life of topical minoxidil is about 22 hours, so once a day is sufficient. I have a compounded spray with 10% minoxidil, dutasteride, tretinoin and a few others that I apply about an hour before I shower for the day which works quite well.
Also, one of the issues with minoxidil is that it is a prodrug, the active form is minoxidil sulfate, and some people don't express sulfotransferase as much in the skin to be able to make that conversion. If minoxidil doesn't work for you, then you want to either use oral minoxidil (the liver will activate it to minoxidil sulfate, the side effect of that is you will have enhanced hair growth through your entire body), or a compounded topical that has tretinoin, since that helps upregulate sulfotransferase in the skin.
> the half life of topical minoxidil is about 22 hours, so once a day is sufficient.
I don't have an opinion on how often minoxidil should be applied, but your reasoning here is wrong. If the treatment works when a particular concentration is maintained, you could absolutely need to apply a drug with a half-life of 22 hours twice a day.
For example, many anti-depressants have half-lives of a few days, but patients still take them once or even twice a day.
The only relationship between half-life and daily dose is initial ramp-up vs sustained treatment. That is, a treatment usually has a targeted amount of a substance being maintained in the body during the sustained treatment, and then the ramp-up and any adjustment needs to be done taking into consideration that the pill you took yesterday may still be 3/4 still in your body today.
Also, one of the issues with minoxidil is that it is a prodrug, the active form is minoxidil sulfate, and some people don't express sulfotransferase as much in the skin to be able to make that conversion. If minoxidil doesn't work for you, then you want to either use oral minoxidil (the liver will activate it to minoxidil sulfate, the side effect of that is you will have enhanced hair growth through your entire body), or a compounded topical that has tretinoin, since that helps upregulate sulfotransferase in the skin.