Ibexa DXP has allowed multiple user content types for a while now, but there is no documented, native way to change the user content type used for registration based on the siteaccess.
As it turns out, it is quite easy with a bit of tweaking.
The EzSystems\EzPlatformUser\ConfigResolver\ConfigurableRegistrationContentTypeLoader
service is used by the registration process to provide the user content type used for