Recommend you check your right of offset. If you offset against the other account you may waive your right as a bank to collect from the customer by other legal actions. Not good if you offset against a balance not sufficient to pay the overdraft.
What do the terms of you account agreement and disclosure provide?
If you return checks on the offset account as there are no longer funds there to pay legally issued checks, make sure you don't return them as NSF.
Think about waiving any charges to return checks on the offset account. You may just be creating another overdraft balance that will need to be charged off.
Be prepared to be contacted by the customers attorney even if you have the right to offset.
What is the cost of potential legal action against the amount of the overdraft loss.
Just some thoughts