If making the lengths of the various conductors 'of equal length', then introducing a percentage of differences will cause long runs to compound whatever issues the requirement is attempting to mitigate. I dunno.. induction heating, out-of-phase, whatever.

Whatever the requirement is trying to avoid, if one conductor is 120 inches and the other 121, then electrically it should be the same if one conductor is 1000020 inches and 1000021 inches.

Neglecting terminal contact resistance, the wire resistance is proportional to length, and current will divide inversely proportional to the resistances. It is the percentage difference that matters, and not the incremental difference. The same incremental difference has a lot greater impact on shorter length circuits, than it does on long distance circuits.

In situation 1, set 1 being 120 inches long, and set 2 being 121 inches long. Suppose it is a 400A circuit. This means the 120 inch conductors will carry 200.83A, and the 121 inch conductors will carry 199.17A. This is what your calculation will look like: 400*(1/120)/(1/121 + 1/120).

In situation 2, the percentage difference is a lot less. The 1000020 inch long conductors would carry 200.0001A, and the 1000021 inch long conductors would carry 199.9999A. At that point, manufacturing tolerances will have a much greater effect, than the fact that the conductors are an inch different in length.