As a simple principle, clearances should be fail safe wherever and for as long as possible. This was hammered into me from day one of training and has always seemed sensible. To clear two aircraft to the same holding fix at the same level just adds another opportunity for it all to go wrong.
Don't worry about the probability of multiple radio failures - it takes far less to distract the controller. And as Defruiter says, multiple radio failures do happen - I suspect I have watched the same radar recording and it wasn't fun imagining how the controller felt.
'Anticipated separation' and all the other examples of 'It'll be OK, I'll keep an eye on it' seem to have featured disproportionately in incidents that I have been involved in reviewing or investigating.