The double spiral staircase of Graz is an architectural element in the former town castle (Grazer Burg). It dates back to around 1500. That way, it is considered the second oldest spiral staircase of its kind.
Decisions, decisions, which one should I take? Whereas in German architecture literature, such a stair is called Zwillingswendeltreppe (twin spiral staircase), tourist resources prefer to speak about a Doppelwendeltreppe (double spiral staircase).
Another known name is Versöhnungsstiege (Staircase of Reconciliation). If you climb up as a pair, you may part on one floor and rejoin on the next floor. The geometry of these stairs makes this happen. This article ofAtlas Obscura reveals a view up to the stairs from below: photo article.
Tip: The entrance to this staircase is inside the inner yard of Graz Castle (Grazer Burg). The door behind the big bush depicted above is generally open to the public. The following movie shows details of the staircase and what to see around the castle: Youtube.