Simple GeometryG 15

Make sure you have the rocket launcher equipped in your inventory, then select it from the weapon wheel.

Best way to get a double kill is to target an occupied enemy vehicle, which usually have at least a driver and a gunner. You can wait by a major road and one should drive by eventually. Or if you’re attacking a base and reinforcements are called, you can target a car full of enemies as it arrives.

Press and hold LB to aim the launcher, then fire with RT.