CB ESA and IR ESA

I see Gary mentioned in post to someone about managed migration to UC that “some people receive both IR ESA or CB ESA”. Although on all letters regarding my son’s ESA it always says he has IR ESA, I remember that when he was migrated from Incapacity Benefit to ESA in 2011 he was put on CB ESA, and it wasn’t until 2014 that he was put on IR ESA and was paid the two disability premiums back dated to the date of migration to ESA. At one point a DWP person told me verbally that my son has both CB ESA and IR ESA - so what happens then with the managed migration to UC if that is the case ?