Create Drupal user for contact joining as Organisation
In the database table that links CMS users to CiviCRM contacts, we store a few things:


The CMS user ID (in Drupal's case, the "uid")
The CiviCRM contact ID
The email address that both records have in common.


Crucially - all three must be unique!  For instance, you can't have a single CiviCRM contact linked to two Drupal user accounts.

The error you're seeing says, "Hey, Civi tried to create a link between a Drupal user and a Civi contact, but we found that another record in this table already uses that same email address."

Based on that - I'd guess that something about your setup is causing Drupal to try to create two user accounts at once - one connected to the individual and one for the organization.

Without knowing more about your setup, I can't tell you the exact steps to fix it, but hopefully this will make the solution clear to you.

One last thing: In CiviCRM, there's a philosophy of not creating "organization" accounts.  "Organizations" can't log into websites; individuals can.  So if what you're trying to do (it's not 100% clear from the question) is create two logins, consider instead using the relationship permissions to allow the individual to edit the organization instead.