I think you misunderstood. Best put Rockwool up to joist height then
lay Kingspan over the joists, this prevents thermal bridging via the
joists.

Wouldn't the boards get quite squashed where they lay on the joists?
Obviously, crosslaying is the best solution. However, if the OP is only
going to fit stuff between the joists, it may as well be Kingspan than
rockwool.

Christian.