It's very likely that the two barrels will shoot to different points of aim, especially if the slugs are lighter than a standard shot load.
The two barrels of a double barrel have bore lines that converge, the breeches are farther apart than the muzzles. This is to compensate for the fact that the top barrel has more muzzle jump than the bottom barrel does when it fires. Shoot a lighter bullet than the gun was regulated for and the top barrel will shoot lower than the bottom barrel. Shoot a heavier bullet than the barrels were regulated for and the opposite happens, the top barrel shoots higher than the bottom barrel. The built in bore convergence is perfect only for one bullet weight.
On the other hand, you can load a slug in one barrel and shot in the other and be "ready for anything".