You can have the following setups:
Picture one, right hand side:
Connecting one BU to two Orgs: Not Supported.
The followup question is probably moot then ("does not apply"), but anyway:
No matter your configuration - unless you have two completely separate Marketing Cloud Instances - you already have testing and live data in the same environment.
Case in Point: All Subscribers List.
Every send to a contact from all BUs will add them to the same list. The "Subscriber Filter"* feature only covers the UI visibility, but still all subscribers from all BUs are physically put into the same All Subscribers list and all can be retrieved from at least one BU (namely, the original parent or Enterprise BU), e.g. via SQL or AMPScript Lookups.
Through the ENT. prefix, other BUs can potentially access it as well - this part might be limitable through a Business Rule though, I am not sure.
Other Example: All Contacts. Multi Org or not, you have one shared All Contacts List, and one contact count. In your current setup, both test and live contacts count against your contact count.
There is no complete data isolation within a single marketing cloud instance, multi-org or not.