You note here that this is the first time you've shot a double rifle

Hold is critical to getting a double rifle to shoot properly.

Did you shoot from a rest?
Did you use a Lead Sled?
Did you shoot off bags?

Anything that isn't a proper hunting hold on the rifle can affect (greatly) the rifle's performance on the target.

Bullet fit VS groove diameter can affect the internal Ballistics. What is the groove diameter? What size did you size those bullets?

Can you describe how the printed target looked?
Shots crossing?
Groups apart?
Good groups from each barrel?