We roll to hit for the first shot and then roll double shot. If the Skill roll for double shot is a success, roll for the second shot. Only if the attacker succeeds with the double shot roll he is allowed to fire twice.
We do allow this skill for short bursts with automatic rifles too. We have introduced a new skill, "Controlled Burst", to allow a character to shoot two bursts in one action.
We have done it the way Torb wrote but we are currently expeimenting to find a way we really like.