that supplied chauffeur-driven cars on the one hand, and self-drive rental vehicles on the other. Both businesses are separate, but associated.
You could have one website with say a domain name like cars.com and then create separate websites within the main site - the correct addressing protocol after the www part would be: chauffeur.cars.com and rental.cars.com - each accessible via links from the main site, or by typing a separate address into a browser. What you would be doing is using sub-domains. The sub-domains would be totally distinct sites, with separate index pages, navigation systems, etc., but wouldn't be hosted separately, on their own domains.