Don't forget also that there is a risk that if formations lose visual contact in IMC then they might split to several levels. Therefore it is prudent to give 2000ft vertical separation.
Unless standard operating procedures have changed, what you have described is
not what would happen if the formation lost visual with each other.
Using 2000' is not a particularly good reason for this event... what about if the formation is a nine ship? How many levels of separation should we use then, just in case they lose visual reference?!!
Notwithstanding Mancboys post, there are standard laid down procedures for breakup of formation in poor vis, we as ATCOS should not start controlling any differently 'just in case', otherwise we would never get our jobs done... there is always a just in case with every instruction we give. We use 1000' separation, but what happens if an a/c has the wrong pressure setting - should we start increasing vertical separation for climbing/descending a/c 'just in case'?!!