Shopify only allows us to make a single call at a time adding one item at a time. Here's a method to add multiple items to the cart at once which works around that.

addItems.js

// add multiple items to the cart by passing an array of item objects
function addItems(items, callback) {
  if(!items.length) {
    // we ran out of items
    if(typeof callback === 'function') callback();
    return;
  }
    
  $.ajax('/cart/add.js', {
    type:'POST',
    dataType: 'json',
    data: items[0]
  })
  .done(function(data) {
    // remove the item we just added from the array
    items = items.slice(1);
    // recursively call the method passing the updated items array
    addItems(items, callback)
  })
  .fail(function(a,b,c) {
    alert('error');
    console.log(a);console.log(b);console.log(c);
  });
}

// example usage
var items = [
  {
    id: 1056086132, 
    quantity: 1,
    properties: {
      chris: 'needs to get a life >:['
    }
  },
  {
    id: 1901920231,
    quantity: 2
  }
];

addItems(items, function() {
  alert('done');
})