They were common, and to some extent, still are, in Germany and Austria, in other places, not so much. Far more common are simple combination guns - a single shot barrel and bullet barrel. Also popular are guns that are not common in the North Americas - the so-called kiplauf - single barrel or over and under guns opening like a shotgun - kip means tip. Even Les Frogs and Eyetalians make and sell them.As noted, many of these had rifled barrels not only in long-dead European calibres, but sometimes calibres unique to the outfitter's store from which they were originally bought. That way the seller would be guaranteed the continuing custom of the buyer. Values can vary between a few hundred dollars to many thousands, especially with names like J P Sauer u. Sohn or Gebr Merkel on the barrels, which, I note you have not shown us the top of. However, as c3shooter advises, every detail of the calibres and even chokes, powder charges and weight of shot and bullet and load for the cartridge will be found on the barrels and action flats, called, over here in UK, the water-table.
There appears to be extreme wear on the 'doll's head' section on the lock - clearly visible between the two firing pins. This integral part of the lock is usually seen totally flush with the surrounding cut-out in the action, think of a dovetail joint that looks loose. Here in UK and any of the other thirteen CIP signature nations that alone would be an instant fail on primary viewing if the gun had been present for Gun Proof at a Proof House.
There is also a degree of justifiable concern about the state of the left barrel. The check out the soundness, hold out your index finger and suspend the group on it by the lug and with it hanging down, lightly tap the barrel down by the muzzle with a wooden kitchen mallet or similar. If it rings clearly, then you have a good group of barrels - if it goes boink/thud or similar non-reverberant resonance, or rather lack of it, then you have big trouble.
Without seeing any other details, I would put a date to this gun of around the early to mid-90s of the 19thC.