Athlon 64's lack the 2nd hypertransport link that would be required for inter-processor communication. That's the same thing that prevents Opteron 1xx chips from working in dual processor configurations on dual-socket 940 boards.
Face it, there are simply no dual-socket 939's and there never will be. Doing so would require plenty of changes to both the processor and motherboard architecture, which is basically what AMD is doing with 4x4 on socket AM2 (and only socket AM2). And even then you'll need to buy a new type of processor designed specifically for dual-processor 4x4 configurations.