Need Help maintaining multiple databases for a multi-tenant application?
You should store all information that is common for all companies in one database and copy the data that are needed to the database that needs the information.
Adding a new customer would be done in the common db.
After that the information will be copied to company1.
If it's needed in company2, then the data will be copied to company2.
Make sure you also add an update process to keep the copied information in sync.
Alternatively, you could keep the customer information in the main db and link to it. However, this would only be recommended if all db's are on the same SQL Server instance.