What is your goal?

if you with share some data i.e. account of chart, I think you should create parent company.

Companies can inherit data from parent company and parent company can summarize data of child companies.

You can create a customer on parent company that is visible on all child companies. Warning: if child companies have different chart of account you must check for receivable account. It is just an example. You meet trouble if child companies have different chart of account. Another big difficult is isolate 2 child companies. You have to look carefully at security rulese and I am afraid you should change some of them.

Check carefully, multi-company has some bug.

Antonio Maria Vigliotti