Hola
Robertoch
Puedes hacerlo así:
Código PHP:
<form>
<input type="text" name="txt1" maxlength="3" onkeyup="if (this.value.length==this.maxLength) txt2.focus()" />
<input type="text" name="txt2" maxlength="5" onkeyup="if (this.value.length==this.maxLength) txt1.focus()" />
</form>
Saludos,
