It's my pleasure to help you in adding a new company, @sarahrenfroe97-g.
You'll have to subscribe to a new QuickBooks Self-Employed (QBSE) account for the second company. You'll have to use a different email when signing up for a new one.
One QBSE/QBO account is for one company file. You may manage branches/divisions by utilizing the Class/Location feature available in QBO Plus and Advanced. Otherwise you should open a new account.