Hi lise-russell,
Great question! I see where you're coming from and hope to help clarify the situation.
In QuickBooks Online, it's only possible to have one company per QuickBooks account. While you can have several accounts related to one set of login credentials, the companies themselves are separated into individual subscriptions and accounts. This is because each company would need its own books and tax information, as Malcolm Ziman has mentioned in his response.
I recommend speaking with an accountant if you're feeling unsure about whether the two companies are separate or not. You can even invite your accountant as an accountant user after getting set up!
Feel free to ask more questions!