Hi Stephen,
A secondary/Destination volume cant be member of more than 1 dataset. The reason being the retention and schedules of a relationship is managed at a dataset level
So if a secondary volume is part of more than 1 dataset there would be conflict in terms of retention settings and schedule time.
In a way, you answered your own questions. If you want to have different retentions and schedule times you will have to have different dataset so separate destination volumes
If the retention and schedule times are same then you dont need multiple dataset.
Also note having more number of relationship terminating in the same destination volume will lead to longer wait time for snapshot creation as all relationship needs to be in Idle state before snapvault snapshot creation.
Regards
adai