But wouldn't be allowed under the franchise agreement as the contract with the DfT requires that trains can run without a second member of staff on board.
There is nothing that
requires Northern to run trains without a second member of staff, it
allows them to do so, although even this is subject to doubt.
It defines DCO as being the "operation of a train by a driver alone without the need for a conductor". The second member of staff can't be involved in the operation of the train, but this says nothing about whether or not a second person should be on-board.
In the DCO section it requires Northern to plan for a second person on board and use "all reasonable endeavours" to operate services according to the plan. This does not require them to run trains without a second staff member, it allows them to.
However there is another section hidden away in the reservations section, which states that on Northern Connect services, passengers must be able to buy on-board seat reservations from a "train host". While I'm a little puzzled by the concept of on-board seat reservations, it implies there must be a second person on-board.