Identity disputes must be avoided as far as possible. This could be maybe done by sharing user-table between the databases. Thus registering in one Wiki would automatically create an account in the other. When disputes arise the needs of Signal Wiki overrides those of Noise Wiki
This does not resolve how Usernames taken elsewhere could not provide confusion by someone hostile registering as themselves as User:Jimbo Wales or User:Fred Bauer for example to make trouble. Though these would naturally be moved by sysops to their traditional owners.
What problems does this present?