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starting from scratch, you need a bog standard MIDI cable.

then you need to setup one device as master, the other as slave.

to do so, open GLOBAL > MIDI > SYNC.

for the master device, set SEND CLOCK and SEND TRANSPORT to YES.
for the slave device, set RECEIVE CLOCK and RECEIVE TRANSPORT to YES.

in MIDI > PORT CONFIG, make sure that the master device has OUTPUT set to MIDI or USB+MIDI. the slave device must have INPUT set to MIDI or USB+MIDI.

connect the master device MIDI OUT to the slave device MIDI IN.
if you press play on the master, both devices should start, and the tempo should be synced.

if you want to sync pattern changes, you need to enable SEND/RECEIVE PRG. CHANGE, respectively.

you’ll need to do one more thing. in MIDI CHANNELS, turn all the channels except the PRG. CHANGE channel to OFF, otherwise the master device will send knob twists etc. to the slave device (unless this is what you want it to do, there is space for experimentation…)