While the Carian Greatsword can only be infused with a single sword at any given time, it is possible to duplicate it so that you can use it on multiple swords. This can be achieved by providing the Blacksmith with the Lost Ashes of War and exchanging it to duplicate an already existing Ash of War in your inventory. Lost Ashes of War can be found in various locations and can be purchased from several Isolated Merchants located across the Lands Between.