Useful strategies include:
Shoot one weak enemy worm with the first shot, and another with the second.
Shoot a weak enemy worm so he is propelled next to a stronger worm. When the weak worm dies and explodes, he does damage to the strong one. Then you use the second shot to finish off the strong one.
Shoot a worm so the blast carries him over a ledge, then take a shot at another target.
Use the first shot to hit a crate or an oil can near an enemy worm. If the resulting explosian doesn't kill him, finish him off with the second shot.
Blast a hole in the landscape with the first shot to give yourself a clear second shot at an enemy.
Shoot the landscape out from under an enemy, causing him to fall.