Spark plug wire routing

67Dart is correct.
There are a number of factors to consider. There is the thickness of the wire itself; 10mm is better than 7mm. Things such as the electrode gap, rotor gap, all contribute to the reqd voltage to ionise the plug gap. If these things cause excessive voltage build, you create the 'perfect storm', then the coupling of wires close together might cause a spark in the close-by wire.
From MSD Tech Bulletins on adjacent Cyl Crossfire: ' The cause is the type & routing of spark plug wires...'