One company file in QuickBooks Online (QBO) is equivalent to one subscription. Thus, you need to sign-up for another QBO account to create a second account for your business. The good news is, you can use your existing Intuit ID or login credentials for multiple accounts.
Once done, you can simply switch from one company to other quickly. See this link as your guide: How do I switch companies?.
If you have any other questions about setting up your second company file, let me know by adding a comment below. I'm more than happy to provide additional assistance. Keep safe!
Yes, you're right, since one company file is equivalent to one subscription. If you want to add another account to your QuickBooks Online (QBO), you can follow the steps provided by my colleague @Angelyn_T.