@valentina To expand on what @jay_s_uk has said and IIRC, by default all the drivers on expansion boards (Duex) are labelled as En. So if you have Duex5, the drivers are labelled E2 to E6. But they are referred to by their driver number. The first 5 on the main board are drivers 0 to 4 and by default these are X,Y,Z E1 and E2. The drivers on expansion boards are thus drivers 5 and 6 (Duex2) and drivers 5 to 9 (Duex 5). But as I said, they are labelled E"something" (at least they used to be when I had a Duex5).
So if you want to use (say) the first driver on the expansion board for another Z motor, then you would map it using M584 something like Z2:5 which would use drivers 2 and 5. The physical labels will likely be "Z" on the main board and "E2" on the expansion board.