DKB is a special case: The bank only transmits checking accounts via "FinTS PIN/TAN." Via this interface, the 2-step authentication is requested by the bank only once every 90 days, either through the old DKB-TAN2go app or via the chipTAN procedure.
The credit cards are retrieved by Outbank directly from the online banking website via the so-called 'screen scraping'. The bank requires 2-factor authentication for every login via the website. Therefore, the 2-step-authentication must also be completed for every account refresh via the blue DKB app.
This means that you still need both DKB apps for strong customer authentication via Outbank.