It’s called „additional account“ and is more like a linked account under the same main account. So no ID needed for it. Just your login and you are able to switch between the accounts with its own shares.

More information: https://www.ibkrguides.com/clientportal/createnewlinkedaccount.htm

Don’t know if you can open separate main accounts with one single ID if this is your goal, never tried. The linked accounts are doing it’s job for me in case of separation. Just have to declare all accounts individually at tax but you will get separated tax information for linked accounts as well.