FYI - I recently spoke to the Albany County Clerk about co-registration of the same (or multiple) guns on two permits.
I explained that I have a father who has many handguns of all different calibers on his permit. We have always been concerned about what to do when he passes away, among other things (i.e., why should i have to buy my own 45 when my dad has two but is not around to go to the range???) The Clerk said that all I have to do is file an amendment, circle "Acquired" and "Other", write in "Co-Registration" and then list each gun that is on my father's permit. Two weeks later, you get a notice to appear in front of the judge, attend the hearing, briefly state the purpose and await for (hopefully) positive decision.
This allows you and a family member (do not have to reside together - I asked that question), to carry and possess each others guns. However, while it permits you to carry and possess, it does not create any ownership rights.
Anyways, I have seen this question elsewhere and wondered myself. The Clerk was very clear that it is permissible and that it is done often. There did seem to be a restriction to family members though.