They're called a Bergstutzen. That sort of combination option you want is sometimes referred to as "Grosskaliber" does exist. The big calibre is usually a 9,3x74R, (which is in the .35 Whelan class. It is a rimmed version of the 9,3x62 which has a good reputation in Africa and is popular in Europe. Several makers produce it. There's loading data in Noslers previous manual). The small calibre varies from rifle to rifle; for instance, .308 Winchester, (An Adamy Suhler), 5,6x50R, (a Blaser B750/88), and .30-06 (Merkel B4) are all for sale on Egun, (a German language website). These are used rifles.

Beware that some other combo rifles, (not listed above), have the large calibre as an insert barrel in a shotgun tube, so you can take it out and shoot 20g shot/slugs too. Only specially strong actions permit use of such a heavy rifle insert.

Because scope objective bells are often large these days, the old idea of looking through the bottom of the mounts and using iron sights, is mostly gone. But that's not to say you might not get lucky. Some are still around. Make sure both front and rear mounts have this see-through. Of course, they quick detach mostly, so this is not so important. Frankly it is also slow to open sight through the small narrow hole.

Not so popular now, but you could once buy a leather scope tube that you could sling across your shoulder using its' shoulder strap. The lid was strapped on so that you could fairly quickly get rid of your scope without having to necessarily get it's case out of your pack, worrying about the lid falling off etc. But if a quality scope has a good range with low-power available, this probably makes iron sights redundant anyway. With the scope on say 2.5x you can pick out which eyeball you want to shoot your bear through, rather than shoot with irons into a dark centre mass inside the tree shadows and hope.

You know, another option (using just one rifle), could be to have nickel plated ammo with heavy bullets, brass cases with light, sight in for the light and know the difference for the heavy stuff which, at short range might not matter too much. The different cases are to make sure you don't shoot your bear with the sheep load and visa-versa.