Edit in JSFiddle

function iFrameLoaded(id, src) {
    var deferred = $.Deferred(),
        iframe = $("<iframe class='hiddenFrame'></iframe>").attr({
            "id": id,
            "src": src
        });

    iframe.load(deferred.resolve);
    iframe.appendTo("body");

    deferred.done(function() {
        console.log("iframe loaded: " + id);
    });

    return deferred.promise();
}

$.when(iFrameLoaded("jQuery", "http://jquery.com"), iFrameLoaded("appendTo", "http://appendto.com")).then(function() {
    console.log("Both iframes loaded");
});

//Note: Prevent focus from getting to Firebug Lite console
Firebug.CommandLine = null;

/*
.hiddenFrame {
border: 0px none; height: 0px; width: 0px;
}
*/