even if you hardi over the lot you still have two substrates underneath which will move seperately.I cant see the hardi solving the movement issue imo
Just to stabilise the different substrates, not to do away with the expansion joint.Why would you want to plug and screw the hardi ?
I have never used it but I would thought one would stick it with tile adhesive to a solid floor. even on a wooden floor ?Just to stabilise the different substrates, not to do away with the expansion joint.