So you're challenge here is to present a scenario where the Weimar democracy survives the turmoils of the 20s and 30s and is involved in an alternate Second Great War, but instead of fighting against some inexplicably super expansionist Soviet Union like is usually presented in these types of scenarios, but is instead, at least loosely, actually allied with the USSR against France/Italy/Britian/ect.
My best guess for a PoD to get something like this is a successful partitioning of Poland between the young Soviet Union and Germany in 1920, which I would assume would both be a legitimizing factor for the young democracy in Germany and help strengthen pre-existing ties between the two powers, for purposes of undermining the Versailles Order.
My best guess for a PoD to get something like this is a successful partitioning of Poland between the young Soviet Union and Germany in 1920, which I would assume would both be a legitimizing factor for the young democracy in Germany and help strengthen pre-existing ties between the two powers, for purposes of undermining the Versailles Order.