Doubling-up fitting and assembly having two doubled-up plates

A doubling-up fitting, comprising:

• a clamp element with a clamp base and at least two clamp arms which rise from the clamp base at its respective ends, the free clamp ends of which are angled inward and extend over the clamp base;
• a rotatably mounted spreading disc arranged in the clamp element between the clamp base and the free clamp ends, with a disc opening leading into a recess in the spreading disc which is open toward the clamp base and the diameter of which is greater than the opening diameter of the disc opening, and with a plurality of outer contour portions which each interact with a respective clamp arm and spread the free clamp ends radially outward less far in an initial rotational position of the spreading disc than in a final rotational position;
• and
• an adjusting disc arranged in the recess of the spreading disc and having a thread;
• wherein the outer diameter of the adjusting disc is smaller than the diameter of the recess in the spreading disc and larger than the opening diameter of the disc opening.