prova così

bt1.onRollOver = function() {
if(varUno == undefined){
movDec(clip1_mc,3,0,0);
loadMovieNum("uno.swf", 3);
varUno = true
}else{
trace("già caricato 1")
}
};

bt2.onRollOver= function() {
if(varDue == undefined){
movDec(clip1_mc,3,-720,0);
loadMovieNum("due.swf", 4);
varDue = true
}else{
trace("già caricato 2")
}
}