Dual FX57's are definitely not possible, as they are single core only (s939), and can not communicate with other s939 processors, they are made as stand alone. So to have dual FX57's you would need two physical processors, the problem is they are built only using one HT link, this means that it only has one HT link to communicate as the northbridge.
The opterons on the other hand have more HT links, therefore they have the capability to communicate with more processors via their extra HT links (AKA single core SMP capable) .. this is in the form of the 2xx series being able to communicate with two opterons, and all the way up to 8 way SMP communication for the 8xx series.
The only SMP capable processor on the socket 939 platform are the X2's, as they are dual core (two cores on the same die) ..
If you wish to use opterons you will need a server/workstation motherboard, the s939 is not for a server/workstation motherboard.
The simple answer is s939 is single socket form factor, and is not built with the ability of having two sockets on the same mobo, that is where s940 comes in.
