Chambering with single point tooling is being used to do rifle barrels. Want to change the neck diameter or freebore? Reprogram the machine. No need for throaters or reamers with different neck sizes.
Haven't read through all the responses but shotguns are fundamentally different than rifles, with regard to regulation, and jointing (and other, less obvious reasons). Shotguns are not regulated by altering the convergence of the barrels (the barrels do converge, of course, but close is good enough). Shotguns are regulated by honing the choke and testing for pattern (the customer will specify how many pellets within a 30" circle at 40 yards). Remember, you're not trying to land two bullets within an inch of each other but rather trying to control a broad pattern (strangely analogous to the uncertainty principle...you can either specify location or momentum to arbitrarily high precision, but not both). Rifles are treated in a vastly different manner (unless you were Sabatti, back in the day, filing muzzles to guide the bullet this way or that...ask someone with such a piece how well that worked). This is why double rifle barrels are made the way they are. If it were all theoretical you could, in principle, make up a pair of double rifle barrels as one piece. I'm practice, though, it's just not in the cards.
Perhaps you're right, they just don't want to go there. but they are a firearm manufacturer and a business. So if they could make a double rifle with their new tooling capabilities you'd think they would.
Longthorne for shotguns, as others have mentioned. Here's an interesting podcast interview with Jim Stewart, founder of Longthorne. He discusses the whole process. https://www.theyorkshiregent.com/podcast/038/
I believe that machining double rifle barrels from a solid block would be possible, but you would need the ability to adjust the barrels for regulation, and at that point, I think any benefit of going through that process would be null and void. Proper regulation of a double rifle is measured in .001" increments, and drilling two 24" holes to have them come to that level of precision and then take into account the vast array of variables is virtually impossible in a production setting. Sure, NASA could probably build one, but the amount of engineering for each rifle would make it cost so much it wouldn't be feasible. I've built many many double rifles on various designs, rebarreled older actions and re-regulated both modern and older doubles. Overall weight of the rifle, length of the barrels, the loads, how the shooter holds the rifle, how far apart the bores are at the breech, balance of the rifle to name just a few of the variables that will be different from rifle to rifle all have effect on regulation. Understand the barrels are not parallel, but the muzzles are closer together than the chambers. The reason for this, is to compensate for the movement of the rifle under recoil as the bullet travels down the bore. So the moment the bullet leaves the bore, the bore has moved slightly up and away from center to then be on target. That's why doubles can be so finicky and might shoot a little different from person to person. They should still be close, but a shooter that "strangles" the rifle as opposed to a shooter that holds the rifle more relaxed, will make the rifle shoot lower and the points of impact closer together. Also, heavier, slower moving bullets with more recoil will require the muzzles to be closer together than a light and fast bullet with less recoil. Less movement of the rifle from recoil, and less time in the bore will require the muzzles to be a bit further apart.
