Is it true that NFA items must be stored in such a way that other (non-registered) members of the household do not have access to them?  I.E. locked in a safe that they do not have the combination to?

If so, is there a way to register NFA items in the name of more than one person without going corporate?

NFA items must remain in the registered owner's control at all times.  If you live alone, or never leave the house while other persons remain at home, you can leave them sitting out on your sofa if you so desire.

Otherwise, they're supposed to be locked up anytime they're not under your personal supervision and control.  If someone else in your home has the combination or key to your gunsafe, the NFA items must be further secured within (maybe by a cable lock to which only you have the key) to prevent their unauthorized possession by anyone else.

I don't know of any way to register an NFA item to more than one natural person.