Doublers were originally designed to lower the nic content of TV liquids, without changing the flavour of any given liquid.
If you had 30ml of liquid X at 24mg and wanted 12mg you would simply add 30ml of doubler X to liquid X and wind up with 60ml of liquid X at 12mg.
Some folks are using them to flavour liquids some with success, but that was not the intent of the doublers.