I suppose you can custom order all manner of configurations. I would not be so sure you should machine a chamber to handle a rim and belted .375.... to me it would seem that there may not be enough chamber wall support for the flanged cartridge head if one machines a countersink to accommodate the belt. I suppose that one might make the extractor/ejector work..... one would necessitate both a pawl for the rimless cartridge and an extractor for the rimmed version.

The easiest is to just chamber one barrel for one, and the other barrel for the other. I went with the flanged for each barrel. I don’t want or need to carry two types of ammo for a double. Kinda defeats the purpose....

However, IF the trickery of accommodating both cartridges in BOTH tubes is both possible and reliable, that might be interesting. The pawl may add unwanted drag and friction to the side of the flanged cartridge and make extraction/ejection less positive, and the reliability of positive ejection using only pawls on a rimless cartridge have been discussed to death with no clear answer, but many lean towards distrust of teeny tiny pawls in dusty environments.