Its a fair question, but it overlooks one huge point, one that is commonly overlooked.
And that is that ALL the .45/.410 combo guns are NOT .410s that can fire .45 Colt, they are .45 Colts that can also fire .410 shells.
now, the .45 Colt, in its original state was a black powder round, and the low pressures of black powder can be contained in shotguns as well. SO that looks like it would work ok. BUT, there are other considerations, one of which is that since the advent of "Ruger Only" level loads, the odds that someone would stick a case loaded well above the safe working limits for many shotgun designs has increased a lot.
Another factor, already mentioned is that the gun has to be a .45 caliber bore. .45/.410 handguns are rifled, to comply with Federal law (smoothbore handguns are NFA regulated items). One could make a .45 caliber smooth bore pump gun, in .45 Colt/.410 but there's no point to it. Smoothbore means little, if any accuracy from .45 Colt ammo, and the oversize bore for the .410 degrades its already limited performance to a degree. So, you lose usefulness of the .45 Colt, and gain nothing (if not lose a bit) of performance of the .410.
you would likely be better off with a .410 gun and shoot .410 slugs when a solid bullet is desired.
On top of all that is the difficulty of making a feeding mechanism that will reliably handle shells of such drastically different lengths.
So, you proposed gun would have to be built more heavily than a .410 shotgun, (heavier gun, less appealing to the market), if rifled, would have absolutely miserable performance from the .410 round, if smooth bore you get miserable performance from the .45 Colt.
Also, if you do get past the issues of feeding the different lengths of rounds, the gun still has to have an action long enough to handle the rounds, which means it has to be longer than a .45 Colt only rifle.
Almost literally, you get the worst of all things, and the benefits of neither round. The only .45Colt/.410 long gun I know of is essentially, an oversized revover action with a long barrel, buttstock, and a gas shield to help protect the forward hand from cylinder gap flash. At least, being a revolver, they don't have feeding issues.
Guns like this are seen and sold, more as novelties, examples of what can be done, more than what should be done. Yes, they are functional firearms, and yes, they do work, but they are not as good at either job as a purpose built .45 or .410.