<form>
<select name="foo" onchange="this.form.foobar.value=this[this.selectedIndex].value+this.form.bar[this.form.bar.selectedIndex].value">
<option value="a">A</option>
<option value="b">B</option>
</select>
<select name="bar" onchange="this.form.foobar.value=this.form.foo[this.form.foo.selectedIndex].value+this[this.selectedIndex].value">
<option value="1">1</option>
<option value="2">2</option>
</select>
<input type="text" name="foobar" value="a1">
</form>
Ändra .value till .text om du hellre vill ha listans text. Fältet foobar kan givetvis vara hidden om du inte vill visa det.