If you want to transfer money between accounts, you want to do it in a way that doesn't mess up reports. If you only created two transactions, Actual would have no way of knowing they are a single transfer and can be ignored in reports.
When you create a transfer in Actual, those two transactions are linked and updating one always automatically updates the other. For example, if you typed something into the Notes column it would appear in both transactions.
Creating a transfer
In the Payee field of a transaction, choose the account in the dropdown you want to transfer to/from. You can press Make Transfer if you want to just see a list of accounts and hide other payees.
A transaction in the corresponding account will automatically be created.
If you are importing files, it is recommended to import into a single account and create the transfer first, and then import the second account. This way the transaction in the second account is automatically reconciled to the transfer transaction. If you imported to both accounts first, creating the transfer would create a duplicate transaction.
Deleting a transfer
Delete a transfer the same way you delete any transaction: hover over it and press the X on the left. Deleting a transfer transactions always deletes both transactions in each account.
Transfers are actually just custom payees, so if you want to you can create custom rules for them.