This has the expected effect in Firefox and Safari (haven't tested IE):

$(':checkbox').change(function() {

if($(':checkbox:checked').length == 2) {

$j(':checkbox').not(':checked').prop('disabled', 'disabled');

} else {

$j(':checkbox').removeProp('disabled');

}

});

In Chrome the checkboxes will disable if the else statement isn't there, when it is the code is effectively ignored.

Changing .prop() to .attr() solves the problem. Of course my understanding of .prop() vs .attr() could be incorrect.