I am still learning netbackup and recently I had an issue as I was not able to make both drives write data and only one of drive was being used. I searched a lot but could not find the answer.Today I managed to make both drives write, I want to share this as this might help someone.
Here is what I did:
Under the Host Properties > Select Master Servers > Properties > Global Attribute > Maximum jobs per client > change the value to 2 ( see attached screenshot.)
They are some suggestions that the Maximum concurrent write drives should be also set to 2, however mine was already set to 2 but it was not working until I changed the Maximum jobs per client from 1 to 2, but it should be set 2 as well.