I know virtually nothing about Rheinmetall typewriters, but some standard-size typewriters did have a second set of spools, mounted on the side like yours, which held not the fabric ribbon but a film ribbon for one-time use (like electronic typewriters and the IBM Selectrics). The film ribbon would have been used for when you wanted a very sharp printed page -- official correspondence or perhaps to make an original document that would be copied.

Thanks guys! According to the serial number it was produced the year 1951. And a sticker of Wurth, I guess manteinance, was put there the year 2000! Is quite a long life span for a office typewriter!

Well by the time this is over, maybe you will be the one to put the explanations and details on the internet,

The dual ribbon system was not an "upgrade" the way we think of it now, as something that can be added by an end user; it was an option to be ordered as a new machine. I don't think it has an official name. It was called different things by different manufacturers. Generically, call it a dual ribbon system and be prepared to explain what you are talking about, since most people don't know anything about this.

Replacement ribbons? Don't know if they're still made. Try online auctions, try "carbon ribbon". If you ask at office supply stores, don't say "typewriter" because they think they don't have typewriter stuff and they'll blow you off. A company that made a lot of ribbons was (is?) NuKote so see if they have a website.

I do have a box of 11 of these ribbons left in stock, but they are pretty old - 1980's I would guess.  You might be able to get similar carbon ribbon by breaking open a plastic cartridge intended for a modern electronic typewriter.  These used 8mm carbon film ribbon too. 

I am certainly not a photographer, but here it goes the Rheinmetall big mama.