Sharing observables

Sharing observables

I have two delivery-type forms (with name, address, postcode etc), and a
button saying 'Delivery address is the same as Shipping Address'.
Basically i'm trying to make it so that when a user clicks that button,
the same observables from the first field are then shared with the second
field. How i have approached this (probably very wrongly) is by setting
two sets of observables like so:
self.firstName = ko.observable("");
self.lastName = ko.observable("");
self.phoneNumber = ko.observable("");
self.address1 = ko.observable("");
self.address2 = ko.observable("");
self.city = ko.observable("");
self.state = ko.observable("");
self.postcode = ko.observable("");
self.delFirstName = ko.observable("");
self.delLastName = ko.observable("");
self.delPhoneNumber = ko.observable("");
self.delAddress1 = ko.observable("");
self.delAddress2 = ko.observable("");
self.delCity = ko.observable("");
self.delState = ko.observable("");
self.delPostcode = ko.observable("");
And then i tried to set a function, that when clicked, would share the
shipping details across.
self.sameDelAddress = function() {
self.delFirstName() = self.firstName(self.firstName);
self.delLastName() = self.lastName(self.lastName);
self.delPhoneNumber() = self.phoneNumber(self.phoneNumber);
self.delAddress1() = self.address1(self.address1);
self.delAddress2() = ko.observable(self.address2);
self.delCity() = ko.observable(self.city);
self.delState() = ko.observable(self.state);
self.delPostcode() = ko.observable(self.postcode);
}