Things to remember here are:
Alters are not other people. They are parts of yourself.
So ultimately it is all from you. Dissociative disorders just make the subjective experience of alters feel like they are -not- you. But they are part of you.
Go ahead and have the candy bar. And if you or another part of you doesn't want it, just put it down?
It doesn't really matter what part of you it comes from. It really doesn't.
At the end of the day, candy bars aren't really all that important. If an alter wants a candy bar but can't tell you they can just get one when they are out, or go without. Or just imagine having one. Or they can get louder with their needs in other ways. But if an alter wants a candy bar but doesn't get one, it doesn't really matter at the end of the day. Candy bars are not essential needs of safety or life.