I was under the impression that the double headed eagle was a symbol of the Byzantine (Eastern Roman) Empire. The symbol may still be seen today in countries which follow orthodox christianity: Russia, Serbia etc.

Could the second head of the Habsberg eagle have been adopted in imitation of the great influential empire of Byzantium which was much advanced technologically over the rest of Europe at the time (i.e. as a form of flattery) - or adopted following Austria's increased importance as guardian of Europe against the Turks/Ottoman Empire especially following the fall of Constantinnople in 1453 (i.e. continuing the fight under the same symbol)?