How to smoothly crossfade image patterns of an SVG

I have an SVG. It uses image pattern to fill up:

<svg>

   <pattern ....>
      <image xlink:href="1.jpg"... />
   </pattern>

   ....................

</svg>

I want to change the image from "1.jpg" to "2.jpg" smoothly crossfading it. How can I achieve that?