@Vipes You won't be able to have your parts on two characters at the same time unless you have two of them. Remember that CASTs are nothing more than a head. The bodies, arms and legs are parts that they use to make a body, so in this sense, CAST parts are the same as costumes/outfits for the fleshy races. They aren't ticket items like accessories that need to be used.
I'm sure you know you can go to the salon or your fashion menu to put the parts on, but once they are on, they're on that character until you take them off.
I hope I'm understanding your right, but it seems that's what you're asking about; using those cast parts on both of your characters. Hope this helps.