[jQuery] Validate textbox if another textbox is filled

[ciupaz]

Hi all,
how can I validate a textbox only if another textbox is filled (by the
user)?

<input name="txtControl" id="txtControl "type="text" />

<input name="txtValue" id="txtValue" type="text" />

I'd like to make "required" txtValue only if the user write something
in txtControl.

Has anyone ideas?

Thanks in advance.

Luis