If we are talking about Standard CC characters, then you do Head/Body mix and match with morphs on CC3+ Neutral base.
First you make Head/Body morphs sliders from Create menu for both of your characters.
Then you load CC3+ Neutral base from Character > Load Neutral Base and dial body morph for one character and head morph for the other.
Finally you can save overall skin from either of the original character and apply to newly created mix, change eyes, add cloth, etc...

For Humanoids characters (or mixing CC head with Humanoid body) a lot more involved.
You have to use Blender to swap heads: https://vimeo.com/999931014?share=copy

Hi, 
I've been searching how to swap just the head on a character but some of the videos I've found are like 7 years old and involve sending the character to iclone and as for current version iclone8 doesn't seem to work the same way.
Is there an updated workflow for swapping or saving a head to be applied to a different body in CC4?
Anybody can help me figure this out? 
Thanks in advance!