Maybe every state is different but I've never heard of a primary or secondary disability. I have A and B for impairments and I'm disabled with one or both.
As long as he has one impairment ,he shouldn't have to worry about a review but the secondary diasbility has me stumped.
At the time I filed for SSDI, A was the impairment and B was a work in progress impairment that has worsened over time. They also mentioned another impairment in my approval letter but it wasn't listed as an impairment for disability.