However, the two items must be linked in a way that can be used to resolve disputes if necessary. The dual signature is sent to both the merchant and the bank. The link is needed so that the customer can prove that this payment is intended for this order and not for some other goods or service.
Answer: A simple action by a customer that quickly becomes very complex to execute.
Once registration is done, cardholder and merchant can start to do transactions, which involve 9 basic steps in this protocol, which is simplified. Add in support for a third party loyalty program and fraud detection analysis system, and you have even more data exchanges and application complexities to consider in order for this single transaction to successfully complete.
So is drilling into this level of transaction detail really necessary? You will not get your caffeine jolt and the merchant loses business due to your card being refused, or the EFT network being down, or a processing switch failing due to being over capacity.