Código:
<script> function mostrar(numero) { document.getElementById('numero')style.visibility='visible'; } </script> <form> <select name=11 id=0 onChange="mostrar(1)" > <option > 1</option> <option > 1</option> </select> <select name=2 id=1 onChange="mostrar(this.id)"> <option > 1</option> <option > 1</option> </select> </form>