In an organization I was performing SAP / Informix DBA duties, we had the following types of skillsets represented within the group and two people to fill each position:
1. Basis Administrators,
2. Database Administrators,
3. System Administrators.
For SAP Security and for the Storage Manager Administrator positions, only 1 person was in each of these positions. Their skillsets were cross-trained and handled by either the Basis Administrators or the Database Administrators during their absences.
Hope this helps.