We have a reclamation and the customer has two joint accounts at the bank. Do we debit the amount of reclamation from both accounts or are we only entitled to debit the account where the ACH payment was credited?

you follow the instructions on the reclamation, if it mentions both accounts, you use both. if it is only 1 account, you use 1. if there are not sufficient funds in the account, following the reclamation instructions concerning less money available than requested.