Start, with figuring out exactly what they are and where they were made. Take clear photos, of the rifles, and the markings on them, which will identify the maker's name, and country of Origin (likely Germany, but could be from several other European countries too). Plug those names in to a search engine, and see if that gets you more info to work with. Yeah, post pictures here too!
If you can, remove the barrels from the action, and take some clear pictures of the markings on the flat surface that becomes exposed. May be markings both on the Barrel Lump, as well as on the mating surface of the action. Those will tell what Proof house, or Proof Laws of which Country, if any, were followed, as well as some details of the cartridges that the barrels were meant to use. Possibly Dates of testing as well, or date codes, which you may need to do some digging to translate. Typically, you close the action, remove the forend wood( often a button or lever, sometimes a screw), and with the forend off, and taking care to control the movement, break open the action, and allow the barrels to rotate to the open position, and they should slide off their mounts.
I have seen a fair few Drillings that were bought and brought back by Canadian Forces members while they were stationed in Germany. Most were 12 Gauge side by sides, some 16 Gauge, with a rifle barrel, often a rimmed variant of a much more available Rimless Euro cartridge, mounted below the pair of Shotgun Barrels. They were not particularly valuable guns, as it all goes, despite having cost rather over the odds at the time they were bought. Just sayin'. "Not that common around here", is not the same as "Rare and Valuable".
Per above, you can troll for info all you want, here, but if you are actually pimping them for sale, the EE is the place to put the listing.