You can use the setItems method of the second field to define what options will be available in it.
If you're interested, I've developed a script that allow you do it without having to actually write any code. You can find it here:
You're missing a reference to the field, using the getField method. The "this" keyword usually refers to the document.
See my example above.
Thats it... i was so close to this solution, but i never managed to make a reference to the field. Thanks!