title: Bind an element to two sources
body: <p>I currently have two text boxes which accept any number. I have a text block that takes the two numbers entered and calculates the average. </p>
  
  <p>I was wondering if there was a way I could bind this text block to both text boxes and utilize a custom converter to calculate the average? I currently am catching the text changed events on both text boxes and calculating the average that way, but I am under the assumption data binding would be more efficient and easier.</p>