You can have multiple or two different companies under the same login. However, each company you want to add will be a separate paid subscription. That said, you'll need to sign-up for a new QuickBooks Online account to create another company.
Fill in the necessary information on the Create an Intuit account or Signup page.
Click Sign In or Add another account if you have an existing QBO account and want to use the same credentials.
Follow the on-screen instructions to continue the process.
Once completed, open this link to log-in to your account: http://qbo.intuit.com. Then, you'll have the option to select your companies. Once signed in and want to switch companies, go to the Gear icon, then Switch Company.
For additional reference about adding a new company or switching companies in QuickBooks Online, check out these articles: