Mineral wool on top of fiberglass. Density and compression
Even though the fiberglass is getting slightly compacted by the mineral wool on top; the minor R-value that may be lost to compaction is easily outweighed by the increased R-value of the extra mineral wool. The only adjustment that could be made (and it is a laborious one) is to increase the height of the joists so they are above the fiberglass and support the mineral wool. Just to be sure the mineral wool is being installed at right angle to the fiberglass?