The option to merge two QuickBooks Self-Employed accounts into one currently not available. Then, I suggest exporting your mileage details from QBSE and import them to another account. This way, you'll be able to bring over the milage between two accounts.
Please refer to this article to view how Quickbooks Self-Employed is designed to help you record your self-employed income and expenses, track mileage, and prepare your Schedule C: QuickBooks Self-Employed Overview.
Please let me know if you have other questions related to QuickBooks. I'm always here to help you. Take care!