ämm... I think I don´t got it right, but here´s my answer:

var super_global_var; //You just have to put it outside the funcs

function set_to_10()
{
if(super_global_var!=10){super_global_var=10;}
}

function set_to_20()
{
if(super_global_var!=20){super_global_var=20;}
}