Support Two companies

How can I make Projects for 2 different companies? Do I have to sing-in with two accounts?

The problem is the following:

- I have two companies and need to separate them (expecially by naming)
- I make project for both
- Sometimes I do have the same clients?
- I want an overview over all tasks, over both companies

What should I do there?

Thanks in advance.

Sincerely,
Thomas