As you already have 17.5Tb of storage , and they are in use , then you have to put the 2 license ( the 16Tb and the other 2 Tb ) ,because if you put the 16 only ,then the DSS will not work , so you have to put the 2Tb,
in this time ( the time you need to put the 2 license) it’s the stop time or let say the time that DSS will not respond to you .tell you put the second key .
i recommend if you can tell other people to stop sending data for few minutes tell you can put the serials.