Sequential Purchase Order Authorisation Notifications

It would be useful to have the option on PO Notifications, that require more than 2 authorisers, to be able to choose that they are sent in a sequential order. The second or subsequent authorisations are only sent when the one before has been authorised. If it gets rejected at lower level then there will be no need to send further emails to authorisers higher up the chain. At the moment all of the emails are sent together and it doesn't matter who authorises first, as long as both do.