Mělo by fungovat tohle:

class ViewModel {
    firstName: any;
    lastName: any;
    fullString: any;

    constructor(first: string, last: string) {
        this.firstName = ko.observable(first);
        this.lastName = ko.observable(last);
        this.fullName = ko.pureComputed(() => `${this.firstName()} ${this.lastName()}`, this);
    };
}