I have no idea about the dual fuel for Petrol engines,, but for Diesel engine i am using one for my jeep running the 13B-t engine. The concept of dual fuel is to enhance the combustion of the original fuel. For diesels the max efficiency is 60-70% of fuel injected to burn. This dual fuel technology is best used in turbo charged diesel engines.
The concept is that diesel has low flash point then LPG/CNG. When LPG/CNG is injected in the air intake system of diesel engine it mixes with air and goes to intake to piston top. As these gases are cooler in nature first it reduces the temperature of air intake increasing the oxygen content. Second due to its high flash point then diesel it burns after diesel resulting in a full and clean fuel burn and consumption.
Petrol is ignited with the help of spark plug and so far I have not seen any international kit of petro which runs on dual fuel. LPG and CNG in petrol engines is used as alternative fuel because of it cost and volume advantage.