The kite shield illustrated, although most closely associated with the Normans because of its appearance on the Bayeaux Tapestry, is attested as early as the reign of Otto III from appearance in the Gospels of Otto III. Otto III is the grandson of the Otto I, who is generally regarded as the first Holy Roman Emperor. This is especially true for someone like Hitler who celebrated the Germanic nature of the Empire, as Otto I was the first Saxon Emperor, and ruled a realm not dissimilar to that of the later middle ages and early modern period
This makes the fireplace an allusion to Hitler's First Reich. Although the kite shields are possibly anachronistic by a generation, they are definitely associated with the Saxon Dynasty founded by Otto I.
Although not definitive, the best images I could find on the web strongly suggest that the knight figures are the same image, mirror reversed. If so, they are both representative of Otto I in my opinion.