Hey BigJordo,
Take your solution, and a bottle of distilled water, to the chem lab, or the local drug dealers house, and borrow a triplebeam scale, or those neat digitals. Weigh equal amounts of distilled water, and PG, by weight not volume. That is a 50/50 solution. Not distilled water in one humidifier, and PG in the other. Though the others are right about the rest of your problem.
Later
Dave (A.K.A. Homebrew)