Under the International Fuel Tax Agreement (IFTA) a "Qualified Motor Vehicle" is one that is:
Used, designed, or maintained for the transportation of persons or property and:
Having two axles and a gross vehicle weight or registered gross vehicle weight exceeding 26,000 pounds or 11,797 kilograms; or
Having three or more axles regardless of weight; or
Is used in combination, when the weight of such combination exceeds 26,000 pounds of 11,797 kilograms gross vehicle or registered gross vehicle weight.
As long as your two-axle vehicles are never used in combination with another vehicle and do not exceed 26,000 lbs., there is no requirement to license those vehicles for IFTA. Additionally, because those vehicles do not meet IFTA’s definition of a qualified commercial vehicle they are not eligible to display IFTA credentials. You should not have IFTA decals on these trucks. They do not have to account for fuel purchases under IFTA - they simply pay the fuel tax as part of their retail fuel purchase at the pump.