I am in discussions with a contractor on how to insulate a staggered double-stud wall, 2x6 on the outside and 2x4 on the inside.

We can save a fair amount of money by filling each stud bay with Roxul and a layer of poly between the studs. I see this detail all the time with blown-in cellulose. What is the downside to 2 layers of Roxul in a staggered stud wall assembly?