% http://texwelt.de/wissen/fragen/23968/
\documentclass{article}
\usepackage{xsim}

% new property:
\DeclareExerciseProperty{shortsolution}

% new environment:
\NewDocumentEnvironment{shortsolution}{+b}
  {%
    \edef\ExerciseType{\csname g_xsim_exercise_type_tl\endcsname}%
    \edef\ExerciseID{\csname g_xsim_exercise_id_tl\endcsname}%
    \SetExerciseProperty{shortsolution}{#1}%
  }
  {}

% we'll use a description list for the list of short solutions:
\newcommand\printshortsolutions{%
  \begin{description}
    \ForEachUsedExerciseByType{%
      \GetExercisePropertyT{shortsolution}
        {\item[Short Solution ##3]####1}%
    }%
  \end{description}
}

\begin{document}

\section{Problems}
% set shortsolution through option:
\begin{exercise}[subtitle=Pythagoras]
  This is the first problem.
\end{exercise}
\begin{shortsolution}
  This is a shortsolution to the first problem.
\end{shortsolution}
\begin{solution}
  This is the solution to the first problem.
\end{solution}

\begin{exercise}[subtitle=Another Problem]
  This is the second problem.
\end{exercise}
\begin{solution}
  This is the solution to the second problem.
\end{solution}

% set shortsolution with custom command:
\begin{exercise}[subtitle=Yet Another Problem]
  This is the third problem.
\end{exercise}
\begin{shortsolution}
  This is a shortsolution to the third problem.
\end{shortsolution}
\begin{solution}
  This is the solution to the third problem.
\end{solution}

\section{Shortsolutions}
\printshortsolutions

\section{Solutions}
\printsolutions[headings=false]

\end{document}