stai ancora utilizzando la variabile locale invece di quella globale

codice:
var tmuno = ""; 
function realclosefly() { 
    MM_showHideLayers('fly1','','hide','fly2','','hide'); 
    clearTimeout(tmuno); 
} 
function closefly() { 
    setTimeout("MM_showHideLayers('fly2','','hide')" ,8000); 
} 
function openfly2() { 
    closefly(); 
    tmuno = setTimeout("MM_showHideLayers('fly1','','hide','fly2','','show')" ,4000); 
} 
function openfly() { 
    setTimeout("MM_showHideLayers('fly1','','show')" ,2000); 
    openfly2(); 
}