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.
