Field collection 'add more' and 'remove' hooks?
I managed to solve this with some jQuery. 

Not sure if this is the "cleanest" way to do this, but it works. I hook into the mousedown() event of the "add another item" button,and trigger the other button's mousedown manually.

I also added a delay of 800ms, because of some conflicting events if the 2 mousedowns are too quickly after eachother.

Hope this helps anyone in the future.

$('#edit-field-pressure-calculation-und-0-field-pressure-calculation-line .clearfix .field-add-more-submit').mousedown(function() {
    setTimeout(function() {
      $('#edit-field-heat-calculation-und-0-field-heating-calculation-line .clearfix .field-add-more-submit').mousedown();
    }, 800)

  });