This is helpful, and you solution works well. So your explanation is, I am
certain, correct.
I *think* something else may be at work here as well. Compare the results of
the following two examples (which I generated with ragged-right = ##t to make
the images compact):
\version "2.18.2"
2018-03-12 20:12 GMT+01:00 Arle Lommel :
> % Cannot tweak both ties in a double tie.
> % Lilypond 2.18.2 simply ignores the second one.
> % This example has a deliberately bad example in
> % the second \shape command to make it obvious where it
> % is working or not, but
% Cannot tweak both ties in a double tie.
% Lilypond 2.18.2 simply ignores the second one.
% This example has a deliberately bad example in
% the second \shape command to make it obvious where it
% is working or not, but *no* values change
% the output from the system. The second \shape command
%