Mixing the same amounts at 24 mg & 18 will give you 21mg. Hopefully that should be perfect for you. I've done that for the same reason as you. I needed 24 mg when I started. Now I'm at 18, but I did 21 for a while. When I reordered juice I ordered 18. I mixed some 24 with the 18, until the 24 was gone. Then I vaped 18. This last order, I ordered some 48 mg & some 0mg doubler, that way I can make up my mind as I go along. I can make some at 18 then if I feel like trying 15 or 12, I can add some of the 0mg to make it what I need. You are not diluting the flavor using doubler. The nic juice is $15 for an ounce & the doubler is about $15 for 4 ounces.
I'm trying to bring my nicotine level down, but not really trying to quit vaping. I hope I can vape the rest of my life.