There is nothing stopping anyone from creating such an exchange. But as far as the Nu network would be concerned , it’s another exchange rather than the only exchange. If it’s features are attractive enough people will flock to it.
Regarding having only one exchange that can sell nushares, that’s simply cannot be done … As long as an exchange has access to the client and they have some shares they can sell it. That’s the beauty of cryptos! No body can limit the medium of exchange by enforcing a third party. Any two people that have the client can transact in it.