I am not the author but have been following this thread closely. It uses
the AutoCAD LENGTHEN command on your second pick. Where you pick the
object to be lengthened makes a huge difference. Picking the object nearer
(along it's length) to one end will apply all the length adjustment to that
end and leave the other end alone. If picking the desired end is not
possible for you, then additional programming would be required to adjust
both ends (say equally, i.e. half the adjustment at each end). Or it could
be done proportionally from where the pick is made along the object's
length.
I am interpreting what you have written that you want the source and target
objects to begin and end at the same points, yet I gather that they take
different routes to get there. That would require reshaping the target to
achieve both lengths and endpoints matching. Not a clear task at all,
unless they were both arcs, in which case they would both end up with the
same radius, an easy solution.
Please let us know if my interpretation is close or way off.