What you could do is the following :
// ----------------------------------------------------------
// Primary Typograpy
// ----------------------------------------------------------
var current;
/* Primary Typography - Size */
wp.customize('typography_primary_size', function( value ) {
current.size = value;
value.bind(function(size) {
$('h1, h2, h2, h4, h5, h6').css('font-size', size);
});
});
/* Primary Typography - Face */
wp.customize('typography_primary_face', function( value ) {
current.face = value;
value.bind(function(face) {
if(current.size.get() == 'whatever'){
current.size.set('othervalue');
} else {
}
});
});
Sorry Otto, it’s perhaps a little bit hacky, but it works. The bonus is that you can also trigger the change callback using the set method : current.size.set('othervalue') will trigger your custom css change for h1 to h6 headers.