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From the Deutsche Bahn forum (24th November 2017):
Is it possible to buy the German Rail Flexi Twin Pass in Germany itself at the train station?
[DB]: You can buy the German Rail Twin Pass in the following train station: Basel Bad Bf, Hamburg Hbf, Berlin Hbf, Berlin Südkreuz, Berlin Zoologischer Garten, Hannover Hbf, Heidelberg Hbf, Kaiserslautern Hbf, Bonn Hbf, Köln Hbf, Köln/Bonn Flughafen, Düsseldorf Hbf, Düsseldorf Flughafen, Dresden Hbf, Leipzig Hbf, München Hbf, München Flughafen, Nürnberg Hbf, Frankfurt (M) Hbf, Frankfurt (M) Flughafen, Stuttgart Hbf und Wiesbaden Hbf.
https://community.bahn.de/questions/1503991-german-rail-twin-pass-flexi