Hide the bottom half of the top figure.
Hide the top half of the bottom figure.
Position and scale for best fit.
Parent top to the bottom, (or bottom to the top).
Hide seam with a belt.

That may sound simplistic but it is just that, a very simple solution. Consider what you are trying to do, the amount of work involved and the end result. If a belt or some other accessory is all that is needed to save you several days work then it may be something to consider.

Plus you'll not have to redo all your morphs.

KISS