from Muffwiggler thanks to all supporter
Note: Green part (sync source) sits on one VCO (e.g. 1), red part (sync destination) sits on another VCO (e.g.3)!
As you can see the signal at the sync switch wires is pretty large (signal is a buffered, -5.6V to +14.4V pulse of 2us width with sync, or -9V DC at Q11-Gate when sync is off), and it controls a switch.
The problem is just if the large switching spikes couple from one side of the On/Off switch to the other, than it is still syncing somewhat, even if it should be off.
The large signals may stray to other circuits.
So maybe it is a good idea to run them shielded, especially when they are longer.
They are not "noise sensitive" in a way as a high impedance audio node. As long as the FET (Q11) stays off when sync is disengaged really nothing happens.