Thank you for posting in the Community, @fastdemand.
That is a great question. Currently, QuickBooks Self-Employed can only track expenses, miles, etc. for each business. We cannot track two businesses at the same time within one account.
Since you'll need to manage two businesses at the same time, you can get another QBSE subscription. Please check this link for more details: QuickBooks Self-Employed Overview.
Let me know if you have other questions. I want to help however I can. Have a nice day!