For the next Blue Kenue installer created after 2 May 2025.
The attribute table for a 2D or 3D Line Set will contain 2 new columns: ClosestOtherLine - which is the line number for a line in the set that is closest to the line under consideration and ClosestOtherLine2DDistance - which is the associated distance.
Note - you may use the menu item Edit... Select Item... in order to select a line given its line number, such as the numbers in the new column ClosestOtherLine.
Kindly, Alan