In holding patterns radar separation is not used, rather procedure control. So if an A/C is cleared to FL120 from (let's say) FL160, unless he reports (asked or not by ATC) that a FL is vacated, ATC cannot clear another traffic to that FL.
In a holding:
ATC: BAW123, descent FL120, report leaving FL160.
BAW123: Cleared FL120, will report leaving FL160. BAW123.
[...]
BAW123: Passing now FL160 down to FL120, BAW123.
ATC: Roger, BAW123. Break. AEA456, descent now FL160.
Hope the example helps.