A good looking rig, but I cannot see any secret code in the standard German proof marks to tell where or by whom your drilling was made. Where do you get the year 1926?
Henry Vogt of Chicago does not Google, he might have been the importer or the customer.
As said, a double rifle drilling is more complicated and expensive than the usual double shot over one rifle. Yours has true sidelocks which adds to the cost, along with Kersten bolting (double Greener type crossbolts.) The front trigger is a single set for an accurate first shot with the right rifle barrel.
Chambering in a standard caliber like 9.3x74R and 2 3/4" 20 gauge is a great boost to its interest and value. But I won't guess a dollar number.