@Khalumar Welcome to the forum!
I don’t use self-hosting, so I cannot test this for you, but according to this comment and this comment, what you’re trying to do should be supported if you are using official Bitwarden server software (not third-party software like Vaultwarden).
If you are using an official Bitwarden server and you cannot add a second self-hosted account, then I would suggest contacting customer support and/or posting a bug report (“New Issue”) on Github. In either case, I would recommend that you refer to the two comments I have linked above (as evidence that this is something that is supposed to work).