Schwarzburg-Rudolstadt and Schwarzburg-Sondershausen
The duchies of Schwarzburg-Rudolstadt and Schwarzburg-Sondershausen were in personal union from 1909, when Prince Günther of Schwarzburg-Rudolstadt succeeded also to the throne of Schwarzburg-Sondershausen, until 1918, when he (and all the other rulers of German monarchies) abdicated.
Read more about this topic: Personal Union