Now that you’re all cabled up, open up the MIDI/Sync tab of Live’s Preferences window on the Master computer.
In the list of MIDI Ports, locate the Output that the MIDI cable is connected to, and turn on the Sync switch.
If you’re using the MIDI output port on a control surface or keyboard, you might have to consult the manual so you know which port to use. Some of these devices have multiple virtual ports in addition to the hardware MIDI port. This means that you might see three output ports for a single device and only one of them will actually communicate with the physical MIDI port.
On the Slave computer, locate the Input that the MIDI cable is connected to, and turn on the Sync switch.
Now, take a look in the upper left hand corner of the screen on the Slave computer:
A new switch has appeared! Neat.
Click on the the EXT switch. This enables external synchronization and disables the transport controls on the Slave computer. To get Live to play back, you’ll need to press Play on the Master computer.
In theory, the moment you do this, playback will begin on both computers and they’ll both play at the same tempo.