Edit in JSFiddle

ko.applyBindings({
    fullName: {
        firstName: ko.observable("firstName"),
        lastName: ko.observable("lastName")
    }
});
<div>
    <div data-bind="text: fullName.firstName"></div>
    <div data-bind="text: fullName.lastName"></div>
</div>
<br />
<div data-bind="with: fullName">
    <div data-bind="text: firstName"></div>
    <div data-bind="text: lastName"></div>
</div>