Hi Klaus,
you are right, master data e.g. customer, supplier, materials, services, etc. cannot be separated in standard. Only company/ sales unit/... specific data can have access restrictions, e.g. only you as AP accountant in company A are allowed to see company A Specific Data of supplier 123.
If you want to have further restriction, you might need to think of customer specific restriction.
If the two companies have so less master data in common, the questions if two separate tenants would not be the better set up?
Best regards
Marlene