A situation in which a vehicle operates on two fuels simultaneously. For instance, a diesel vehicle (operating on diesel originally) is converted to run simultaneously on both diesel and Natural Gas in either Liquified (LNG) or Compressed (CNG) storage form.
A situation in which a vehicle operates in alternative fuels. For instance, a petrol engine that was running on petrol/LPG originally is converted to run alternatively on either petrol/LPG or natural gas (LNG or CNG).