Edit in JSFiddle

function getCustomer(customerId){
    var d = $.Deferred();
    $.post(
        "/echo/json/",
        {json: JSON.stringify({firstName: "Jose", lastName: "Romaniello", ssn: "123456789"})}
    ).done(function(p){
        d.resolve(p);
    }).fail(d.reject); 
    return d.promise();
}

function getPersonAddressBySSN(ssn){
    return $.post("/echo/json/", {
            json: JSON.stringify({
                ssn: "123456789",
                address: "Siempre Viva 12345, Springfield" })
    }).pipe(function(p){
        return p.address;
    });
}

$.when(getCustomer(123), getPersonAddressBySSN("123456789"))
    .done(function(person, address){
        alert("The name is " + person.firstName + " and the address is " + address);
    });