I think you have two options:
- Use a reverse proxy so you can even have two different domains for each instead of a path. The configuration for this would change depending on your reverse proxy.
- You can change the config of your pihole in
/etc/lighttpd/conf-available/15-pihole-admin.conf
. In there you can see what's the base url to be used and other redirects it has. You just need to remember to check this file each time there's an update, since it warns you it can be overwritten by that process.