Speakers on top of subwoofer

Is mounting a speaker on top of a sub a bad idea?  Specifically, a Harbeth 40.x mounted with isolators on each corner so the cabinet can flex/breath on top of a Rythmyk E15  I have a limited amount of floor space and the Rythmyk is a little taller than a Tonteger which is a good thing since the Tons are a little short to place the tweeter around my ear height.