Add new subscriber to two lists at the same time

When people join some of my lists, I want to add them to an umbrella list at the same time.

I tried to change the code to the form like this:

1: <input type="hidden" name="list" value="3,4"/> (subscriber was only subscribed to list 3)
2: <input type="hidden" name="list" value="3"/> <input type="hidden" name="list" value="4"/> (subscriber was only subscribed to list 4)

How can I do this?