You might want to assign the same approval amount to multiple persons so that if one is not available to approve an order, another is available who can. Only the first person on the approval route with the lowest address book number will receive a message that an order is awaiting approval, although any of the persons can approve the order. The same person cannot appear more than once in a table.
I wasn't aware that only one person got the email, but we had a very heavily modified approval system.
A fix for that is to make an address with a group email address that sends to everyone required and assign it at x amount, and then all of those people on the list set up at x+1. That way, they all get the email, and when any of them approve it it jumps through to the next level.