Thanks the the replies.
I solved my problem with jQuery.
Here is the code:
jQuery(document).ready(function ($) {
$("#category-tabs li.hide-if-no-js").hide(); //Hides Most Used tab
$("#category-all input:checkbox").change(function () {
var max = 2; // Max allowed cats
var count = $("#category-all input:checked").length; //counts selected cats
if (count > max) {
$(this).prop("checked", "");
alert('You are only allowed to select ' + max + ' categories.'); //alert message when user tries to select a third cat
}
});
});