The tooltip is an HTML property that you won't be able to change, not its behavior at least.
If this behavior doesn't fit you, you should then try to replicate it. Each textBox could be put in a container with a text/spritefont object, so when one is created/destroyed, so is the other. The text would be used as a tooltip then, you just have to position it. You can also add a 9patch in the container to make a background for this tooltip. Then, all you have to add is display/hide the text/BG depending if the textBox is focused or not.