Allow me to take care of this question for you.
The option to merge companies into one isn't available at the moment. If you're looking to transfer files from one company to another, you have the option to do this by hand.
Here's how you can download bank transactions:
Here's how you can export recorded mileage:
If we're talking about a duplicate company, you can choose to cancel one and keep the other.
More on QuickBooks Self-Employed import, export, and conversion limitations are in this article: Self-Employed: Import, export, and conversion limitations. You can learn more on what you can or can't do with your Self-Employed data.
I'll be around if ever you still need help. If you have other questions feel free to put them in the comments below and I'll get back to you.