Edit in JSFiddle

function AppViewModel() {
    var self = this;
    this.firstName = ko.observable("why");
    this.lastName = ko.observable("520crazy");
    //this.fullName = ko.computed(function() {
    //    return this.firstName() + " " + this.lastName();
    //}, this);
    this.fullName = ko.computed({
        read: function () {
            return this.firstName() + " " + this.lastName();
        },
        write: function (value) {
            var array = (value || "").split(" ");;
            self.firstName(array[0] || "");
            self.lastName(array[1] || "");
        },
        owner: this
    });
}
ko.applyBindings(new AppViewModel());

<fieldset ng-controller="simple">
    <legend>例子</legend>
    <p>First name: <input data-bind="value:firstName" /></p>
    <p>Last name: <input data-bind="value:lastName"  /></p>
    <p>Hello,    <input data-bind="value:fullName"></p>
    <div data-bind="text: fullName"></div>
</fieldset>