One business is equivalent to one subscription especially if each business has separate Tax reporting. Let me share some information on how it works.
You'll want to sign-up for a new QuickBooks Online account to for your 2nd business. This way you can generate separate expense and other financial report for it.
Also, you can put these accounts under your existing Intuit ID, log into your current company when creating a new account. Simply click on the Add another company button when creating another account. Here's how to add a second account:
Once completed, you can go ahead and click the Gear icon at the top to Switch Company. For more information, you can refer to these articles:
Keep us posted here if you have other questions or concerns. I'm always here to help.