Compulsory Selection in Multi Select

Thank you so much. Good instructions.  One clarification though. Do I need to add the formula field only once in the table and this could work with different modules.  I seem to have two similar fields with different names but addressing different issues.   

Having some issue. I created the .JS file. I already have the required variables.  I also created a text field with the following code: 

[iol] & "MultiSelect.js" & [/iol]

But when I am saving a new record, it does not fire the error. 

Debug with console.log() statements in your code.

Thanks. Where do you place it? 

$("#_fid_110").bind("multiselectclose", function(event, ui){  var selections = $("#_fid_110").multiselect("getChecked");
  if (selections.length < 2) {
    $("#_fid_110").multiselect("open");
  }
});