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.