JSFiddle - React, Tailwind, and code Playground

HTML

<script src="http://ajax.aspnetcdn.com/ajax/knockout/knockout-2.2.1.js"></script>
<div data-bind="text: TotalAmount"></div>

JavaScript

var Cart = function() {
  var self = this;

  self.Products = ko.observableArray([{AmountIncludingVAT: ko.observable(100)}]);
  self.Products2 = ko.observableArray([{AmountIncludingVAT: ko.observable(200)}]);
  self.Messages = ko.observableArray([]);

  self.TotalAmount = ko.computed(function() {
    var result = 0;
    ko.utils.arrayForEach(
      self.Products().concat(self.Products2()),
      function(item) {
        result+=item.AmountIncludingVAT();
      }
    );
    return result;
  });
};

var cart = new Cart();

ko.applyBindings(cart);