\version "2.19.43"

% Including them one after another works fine
%%{

#(ly:parser-include-string "\\include \"levelOne.ily\"")
#(ly:parser-include-string "\\include \"levelTwo.ily\"")

%}

% Wrapping the includes in a Scheme function doesn't work
% because \music will only be visible when the expression
% has been fully evaluated (i.e. after leaving the function).
%{

includeBoth =
#(define-void-function ()()
   (ly:parser-include-string "\\include \"levelOne.ily\"")
   (ly:parser-include-string "\\include \"levelTwo.ily\""))

\includeBoth

%}


% Using ly:parser-parse-string with a parser clone doesn't
% work either because it seems not to be able to parse
% the (first) file in the first place.
%{

includeBoth =
#(define-void-function ()()
   (ly:parser-parse-string (ly:parser-clone)
     "\\include \"levelOne.ily\"")
   (ly:parser-parse-string (ly:parser-clone)
     "\\include \"levelTwo.ily\""))

\includeBoth

%}