Obviously there is no barrel regulation on these guns. Some people will jam a piece of metal, or a coin between the barrels at certain points, but it doesn't look very aesthetically pleasing if it works at all.
I go sort of the same route Cas did, but with a scope for the rifle barrel set in see thru mounts so I can use the irons for the shotgun barrel. I'm really not a fan of see thru sights, I really don't like them at all, yet I can say they work well, and I find them necessary on a Savage 24, especially on the 24V's(centerfires).
Shown is a Savage 24 Series S in 22 magnum over 20 gauge with see thru NC Star Aluminum Dovetail rings and a Western Field 3X9 32MM scope.