I had 4 rifle/shotgun combos in the past. .222/12G, .243/12G, .308/12g and 6.5R/12G.
They were Savage or Valmet. Missed a 8 point buck standing broadside from 20 yards using the .308/12g. The rifle was the bottom barrel. I fired the rifle and it shaved his belly. At that range the rifle was shooting low. The combo was sighted in for 50 yards. In hind sight I should've fire the shotgun instead. It had a slug. At 20 yards it would've hit the boiler room dead center.
The next year I use the .243/12g. This time the 12 point buck was further at ~25 yards and I shot it with the shotgun with slug. Didn't repeat the same mistake from last year. The buck ran 50 yards and drop. Learn from past experience and from a lot of shooting at the range, under 30 yards use shotgun, over 30 yards use rifle.
They were all finicky to regulate. Prepare to spend a lot of time and ammo to regulate to your liking. Since you are hunting small game, get to know the point of impact on all your hunting distances for both barrels.