I'm not sure that it can be done out-of-the-box, but here is a solution that could get you started.
 

This example takes one Single Line field which has the ID of 3 so you would replace q3 and #Field3 with the correct.

I would also check that that there are no empty fields before allowing to add a second set to the collection.

The second function also makes the previous field non-readonly in case you decide to remove an item from the collection.

$(document).ready(function(){

    $(document).on('click', '.cf-collection-append', roItems);
    
    function roItems() {
    
    var items = document.getElementsByName("q3").length; // this is the textbox

      $("#Field3\\(" + (items-1) + "\\)").prop("readonly", true);

    }
  
  
    $(document).on('click', '.cf-collection-delete', reItems);
    
    function reItems() {
    
    var items = document.getElementsByName("q3").length; // this is the textbox

      $("#Field3\\(" + (items) + "\\)").removeAttr("readonly");
      
    }


});