There's more than one way to do this. The easy way is to set everything except the head and neck of figure one invisible, and the head of figure two invisible; then position them so that they coincide. This may be acceptable for a single image (although you'll probably have to postwork the neck join), but a better solution would be to, basically, make a new figure. This will involve modelling and mapping a new neck to join the head and body together, at the very least.

What figures are you planning to join? Someone may have done it already.