Hi, Gun Ho and guys,
No, the correct German word is "drilling", and pronounced just as we do. The German words for 1, 2, 3 and 4 are ein, zwei, drei, and vier. "Zwillinge" means "twins" and the term is not usually used for guns, doppelbuch (double barrel) being used instead. However "drilling" and "vierling" are used for guns. A drilling has two shotgun barrels, and a rifle barrel in the middle below them. A vierling (pronounced "fearling") is the same, but with a small bore (usually .22 LR) barrel in the top rib.
A German 20 gauge would be uncommon. I think you might find that the shotgun barrels are 16 gauge and probably for short shells. The bottom barrel might be 8mm, but it would likely be for a rimmed cartridge, not the German 8mm military rifle cartridge.
I suggest taking the gun to someone familiar with those guns before shooting it and getting advice on the caliber and how it works.
AFAIK, Hitler did not ban drillings (why?) and in fact many were bought by the Luftwaffe for "survival" guns for shot down pilots. Obviously, few were made for the civilian sporting market after the war began, but I know of no "ban".
Incidentally, there were drillings made in this country, by a Hollenbeck company in West Virginia, now long out of business.
Drillings in any kind of good condition bring some very nice prices today, so definitely don't do anything to it you might regret later.
Jim