You may have imported transactions in your banking feed for both accounts that aren't entered as a Transfer in Quickbooks Online (QBO). If that's the case, you can record it as a transfer from one account. Then you can match the transaction from the other account.
Here's how to do it:
In QBO, go to the Banking menu at the left pane to get to the Banking page.
Select the bank account that the transaction is coming from.
Find and click the transaction to open it.
Tick the Record as transfer radio button. Then, from the Transfer account dropdown, choose the account the money was transferred to.
Select Record transfer and the bank account to which you transferred the transaction.
Proceed to the For Review tab and click the Recognized filter from the drop-down.
Find the transaction. From the Action column, pick Match.
This will match the transaction in both accounts. To learn more about managing and transferring funds in QBO, consider checking out this article: Transfer funds between accounts.