There was an incident in one of the Heathrow holding stacks a few years ago where TCAS had a domino effect in the hold. It certainly was interesting to watch! One of the aircraft at the top of the hold descended to the same level as another, the lower aircraft got a TCAS descent which resulted in the aircraft below it getting the same.
In such a scenario, the best a controller can hope to do is to pass traffic information and give vectors to separate the aircraft horizontally so that TCAS can step out.