Why not check how the double is regulated, then you would now. If not possible take it apart and see if you could make a difference how the sear is stoned (which one is the one with the largest bite). I bet its not the front trigger.

Anyhow dont think just put the rifle to the shoulder and shoot imediatly when it lands on you and as quick as you are able fire the next barrel.

Thats how you should shoot.