Oldramrod's advice is good! I would add that the snugness of your overshot card is influential in whether or not you will have an issue. If you follow O's advice you will figure it out pretty quickly. I have a Pedersoli 20 gauge that I was using in dog training. It became apparent that the load in the second barrel was moving some from the recoil in the first (right) barrel. So i started loading 2 overshot cards, which fixed it. Then, I bought 19 gauge overshot cards from TOW, and only need one card to hold the load tight.
If you have a dial caliper it's worth measuring the actual bores instead of counting on the gauge being precise. Different vintages of Pedersoli 12 gauge vary significantly, from .730 to .708 on my caliper.