This is by design. Only one Recovery Point Set job can be defined per drive. The Recovery Point Set option is not available if drive is already assigned to an existing backup and specified Recovery Point Set as the recovery point type. This option also is unavailable if you select an un mounted drive that cannot be part of a recovery point set.

In such cases where two backups are to be scheduled for the same drive, the second backup job should be configured to create an Independent recovery point set (Creates a complete, independent copy/full backup of the drives that you select).