Not the same material, but I was amazed at how stiff a plywood diaphragm floor, made from two skins of 9mm plywood, could be when I installed a load of it a few years back (far stiffer than a single 18mm sheet would be). The downside to that was the need to use about 300 ring nails per installed sheet and use a very particular nailing pattern. When we fixed 12mm cement fibreboard over the top (screwed at 300mm centres) it seemed to not make much difference one way or the other. Maybe something to ask the S/Es on here?
But I reckon that adding a sheet of cement board to the top of 18mm plywood could potentially increase the stiffness, but from what I've read it would need a similar nailing or screwing pattern to that used for ply overboarding, i.e 325 ring nails per sheet on 100mm centres or about 153 screws on 150mm centres.
Of course the joints would need to overlap and installation would be a bear without a gas or compressed air nail gun (I wouldn't even consider screws TBH)
