Readers digest condensed version (Linux - Ubuntu 15.04)
Machine 1:
steemd --miner='["machine1","5xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"]' --witness='"machine1"' --mining-threads=11 [or whatever]
Machine 2:
steemd --miner='["machine1","5xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"]' --witness='"machine2"' --mining-threads=11
Note 5xxx... is the same for both machines.
Enough said.