Answer
Yes, you can use the same bank accounts to fund multiple investment accounts.
However, please check the correct bank details of the account to which you should transfer your funds. A unique cash account held at flatexDEGIRO Bank AG is linked to every unique DEGIRO investment account. The details of the account can be found by clicking on 'Deposit/Withdraw' and then on 'Manual transfer to DEGIRO'. If your investment account does not (yet) have a linked cash account, please state the username of the target account in the payment description of your manual transfer.