When you want two cameras to have matching timecode you need to synchronise not just the time code but also the frame rates of both cameras. Remember timecode is a counter that counts the frames the camera is recording. If one camera is recording more frames than the other, then even with a timecode cable between both cameras the timecode will drift during long takes. So for perfect timecode sync you must also ensure the frame rates of both cameras is also identical by using Genlock to synchronise the frame rates.
Genlock is only going to make a difference if it is always kept connected. As soon as you disconnect the Genlock the cameras will start to drift. If using genlock first connect the Ref output to the Genlock in. Then while this is still connected connect the TC out to TC in. Both cameras should be set to Free-run timecode with the TC on the master camera set to the time of day or whatever time you wish both cameras to have. If you are not going to keep the genlock cable connected for the duration of the shoot, then don’t bother with it at all, as it will make no difference just connecting it for a few minutes while you sync the TC.
In the case of a Sony camera when the TC out is connected to the TC in of the slave camera, the slave camera will normally display EXT-LK when the timecode signals are locked.
Genlock: Synchronises the precise timing of the frame rates of the cameras. So taking a reference out from one camera and feeding it to the Genlock input of another will cause both cameras to run precisely in sync while the two cameras are still connected together. While connected by genlock the frame counts of both camera (and the timecode counts) will remain in sync. As soon as you remove the genlock sync cable the cameras will start to drift apart. The amount of sync (and timecode) drift will depend on many factors, but with a Sony camera will most likely be in the order of a at least a few seconds a day, sometimes as much as a few minutes.
Timecode: Connecting the TC out of one camera to the TC in of another will cause the time code in the receiving camera to sync to the nearest possible frame number of the sending camera when the receiving camera is set to free run while the camera is in standby. When the TC is disconnected both cameras timecode will continue to count according to the frame rate that the camera is running at. If the cameras are genlocked, then as the frame sync and frame count is the same then so too will be the timecode counts. If the cameras are not genlocked then the timecode counts will drift by the same amount as the sync drift.
Timecode sync only and long takes can be problematic. If the timecodes of two cameras are jam sync’d but there is no genlock then on long takes timecode drift may be apparent. When you press the record button the timecodes of both cameras will normally be in sync, forced into sync by the timecode signal. But when the cameras are rolling the timecode will count the actual frames recorded and ignor the timecode input. So if the cameras are not synchronised via genlock then they may not be in true sync so one camera may be running fractionally faster than the other and as a result in long clips there may be timecode differences as one camera records more frames than the other in the same time period.