Assuming one legal entity for legal reporting purposes I would have one company and use sites and financial dimensions to report profitability etc. Yes there is a security question, but with the majority of services shared having those departments continually logging into separate companies to do what in essence is a centralized task does not seem a sensible approach.
That said it is not really a question you can answer with the information given, you can offer opinions. The initial analysis with your partner as they understand your business will help them to help you to make the right decision here.