Where to start?
When a juice is marked 12mg it actually means 12mg per ml. So every milliliter has a mass of 12 milligrams of nicotine suspended in it. That means 1ml of 12mg/ml mixed with 1ml of 24mg/ml is (12mg + 24mg)/2ml for 18mg/ml.
If you have been mixing 12mg/ml and 24mg/ml in a 50/50 ratio together, you have been vaping 18mg/ml. It could be that 24mg/ml will be too much TH and nicotine for you. If you have increased the power you are using to produce vapor, then the nicotine will be absorbed even quicker and the throat hit will be greater for the SAME nicotine level.
Now add increased power, also known as heat or work, to increased nicotine and you are asking for a nicotine headache.
I would strongly suggest you drive to the nearest Walmart and look in the section with the bandaids and rubbing alcohol and grab a bottle of
http://www.walmart.com/ip/Humco-Skin-Protectant-Glycerin-Usp-6-oz/10417582?adid=bzv_fb_revshr_001 and use it to reduce the nicotine of the incoming juice.
Since the Kgo at 18mg/ml is a bit strong, you should take the 24mg/ml juice and mix it 50/50 with the VG from Walmart and make some 12mg/ml juice to get you by until you can order more. It will dull the flavor, but increase the vapor. More importantly, it will not make you ill and give you the nicotine fix you need. You can add 10% distilled water to the mix if it is too thick for your equipment without a huge impact on the nicotine level.