We are exploring the possibility of having 2 separate business units in one Enterprise 2.0 SFMC account connected to 1 Sales Cloud org via the MCC connector. Is that possible? We don't have issues with having a separate API user for both connections, and the MCC is already up and running, establishing 1-to-1 connections between various SFMC BUs and Sales Cloud orgs.
I’ve had these cases before. We activated the multi org and then connected two BU to the same Salesforce Org using the same API user. And another org, we connected to a sandbox. Maybe you should think about the functionality of the Multi-Org. Be careful, if you enable it, you can't disabled later.