Actually they are not. In Belgium and the Netherlands we also have two figures. We have 'sinderklaas' (Saint Nicholas) who comes on december 6th with presents and then you have Santa Claus on Christmas (who is a lot less prevalent here).
Actually the way I've understood it is that Santa Claus is a hybridization of figures like Saint Nicholas and Grandfather Frost and some others. When immigrants moved to the US, they took their own figures with them and in the course of time a hybrid figure emerged, which is what we now know as Santa Claus.