There are two ways SPO staff can see all proposals in a signing official's queue.
Option 1: View another person's Action List
This option shows items that are already IN their action list. To see items that are going to route to them in the near future, use option 2 (below). OtterIDs for Common Signing Officials available here.
Option 2: Search for SAVED and ENROUTE proposals
This option will show any items where the user you listed shows up in the route log as an approver including ones they've already approved and have gone on to the next signing official. So there is some noise that will appear in this search, but the benefit is that it shows items that are going to route to them in the near future as opposed to just what's already in their action list.