KnockoutJS subscribe to multiple observables with same callback action

Do you not want to duplicate handler function's body? Extract it to a variable.

function CaseAssignmentZipCode(zipCode, userId, isNew) {
  var self = this;
  self.zipCode = ko.observable(zipCode);
  self.userId = ko.observable(userId);
  self.isNew = isNew;
  self.isUpdated = false;

  var handler = function () { self.isUpdated = true; };

  self.zipCode.subscribe(handler);
  self.userId.subscribe(handler);
}

You can use a computed observable for this purpose. You just need to make sure that you access the value of each observable in the read function. Would be something like:

ko.computed(function() {
   self.zipCode();
   self.userId();
   self.isUpdated = true;
});

So, you get dependencies on the two observables and set your flag.

Also, if you are looking for something like a "dirty" flag, then you might consider something like: http://www.knockmeout.net/2011/05/creating-smart-dirty-flag-in-knockoutjs.html. The idea is that you use a computed observable that calls ko.toJS() on an object to unwrap all of its observables.