Do I need an additional storage license key if I connect one DSS 16TB to another DSS 16TB with iSCSI/FC protocol ?

This depends on how the storage will be connected and how much storage space will be visible to the system.

It depens only on how it will be connected and how much storage the system will see because you need as much storage license as much storage space will be available in the system.

For example, when you have one DSS with 16TB storage space connected to another DSS with 16TB local storage, the resulting storage space on the second DSS is 32TB. In this case you will need an additional 16TB storage license key for the second DSS.

If your second DSS does not have any local disks in addition to the 16TB from iSCSI connection, you will only need the standard 16TB license.