The problem of this standard is that it would not allow multiple owners of a token because each token is not divisible.
Is there possibly confusion between the idea of divisibility and fractional ownership? They're quite different concerns.
Divisibility is about division. For example, you can divide $100 into two $50 but you wouldn't want to saw a kitty in two because that would be very bad for the cat. Still, joint ownership of a single kitty is possible.
It's true that ERC721 considers only a single owner. This is a common pattern that keeps the core standard and code compact but it doesn't prevent joint/fractional ownership, if that is what is needed.
The trick is that the "owner" could be a contract. That's up to you at an application-design level. You could put any sort of multi-signature or governance contract in place and give it custody of assets.
Take this specific contract from a multi-sig wallet client as an example:
Notice that owner is set to an
address (array), which can include one, or more addresses. You could limit the max array size to include two addresses, or a hundred, it's up to you.
An implementation is then needed where two parties can send Ether to the multi-sig contract which purchases the NFT; The contract owns the NFT, and in turn any number of addresses in the
address control the contract. Custom logic will be needed to determine how consensus takes place if one party wants to sell, etc.
Hope it helps.