Doubler doesn't work well because it isn't concentrated -- it is designed to mix with other eliquid that already has precisely the same flavor, but higher mg than you want. If you would indeed like eliquid at say 19 mg you could only have 20% of the doubler in the final mix, it would be a really weak flavor.
You need concentrated flavors -- the perfumers apprentice, onestopdiyshop, ...
You need concentrated flavors -- the perfumers apprentice, onestopdiyshop, ...