"demand deposit" is the wrong word - I think the theoretical name is "book money". It is the 2nd of the three money types. It consists of what is basically 1 person lending money and 1 person lending out money. Since both parties have a legal right to resulting balance - which is why it is two databases. This isn't yhe case for the 3rd of 3 types of money where your only claim to the mony is customer protection or whatever thus only one database