There are several scenarios you can consider before merging your two accounts as one in QuickBooks Self-Employed.
First off, if you're adding two accounts for your one bank account, you do so by following the steps below:
Go to the Gear icon.
Select Bank Accounts.
Click Connect another account.
Once done, you'll have to turn on and off a certain account if you want your transactions to be downloaded into that account. Also, here's an article you can read to learn more about adding your bank accounts: Connect a Bank Account.
However, if you're adding two separate bank account as one, there isn't an option to do so. Bank accounts in QuickBooks Self-Employed have its own financial information so you're unable to merge them.