To apply more than one guid the syntax is:
mx.data.action({
params          : {
    applyto     : "selection"
    actionname  : "MyFirstModule.GetFavoriteFood",
    guids       : [281530811285515, 281530811285506],
},
callback        : function(obj) {
    // expect single MxObject
    alert(obj.get("manufacturer"));
},
error           : function(error) {
    alert(error.description);
}
}, this);
You can see all of the documentation at http://apidocs.mendix.com/5/client/data.html