double-preload.js

var preloadCount = 0;
function preloadEvent() {
  preloadCount++;

  if (preloadCount >= 2) {
    // Show the images
  }
}

var image1 = new Image();
image1.onload = preloadEvent;

var image2 = new Image();
image2.onload = preloadEvent;

image1.src = "/images/image1.jpg";
image2.src = "/images/image2.jpg";