I don't think that would work. The central post is not a really strong attachment point above the antenna. It is inserted in the base and held with a couple of set screws. It is not all that structural really. Of course, it is help by the cords, so maybe it is possible. I will have to mull that one over.
No mulling necessary. The center post and hub of the Hex is not designed to accommodate another antenna above it. The cords are only for holding the spreaders in position for the wire elements. You would be better served if you were able to have a taller mast that would allow you to put the other antenna below the Hex.