Edit in JSFiddle

var a = document.getElementById("a"),
    b = document.getElementById("b");

a.addEventListener("change", function(){
    var valor = this.value;
    Array.prototype.forEach.call(b.options, function(opcion){
        opcion.disabled = opcion.value == valor;
    });
}, false);

<select id = "a">
    <option value="1">Opcion 1</option>
    <option value="2">Opcion 2</option>
    <option value="3">Opcion 3</option>
    <option value="4">Opcion 4</option>
</select>

<select id = "b">
    <option value="1">Opcion 1</option>
    <option value="2">Opcion 2</option>
    <option value="3">Opcion 3</option>
    <option value="4">Opcion 4</option>
</select>