There are times you may want to pull data into your ViewModel from multiple sources. When doing this you can create a map for each source. If the source variables have the same name they will overwrite the existing variables. As long as the base variable is a different name it will merge them into the ViewModel. Do this as follows:

myViewModel = ko.mapping.fromJS(firstData, firstMap);
ko.mapping.fromJS(nextData, nextMap, myViewModel);

What you get is a combination of the firstData JavaScript structure, mapped with firstMap, combined with the nextData JavaScript structure, and with the nextMap mapping. If there are any duplicate base structures in nextData, they will override the same structures in the existing firstData JavaScript structure.