Can't make table with non-standard multicolumns

Note if you use vertical lines in multicolumns you should only use them at the left of an entry (except the first entry in the row which can have them either side) otherwise they will not line up correctly.

\documentclass[8pt]{extarticle}
\usepackage[a5paper]{geometry}
\geometry{verbose,tmargin=1cm,bmargin=1.5cm,lmargin=0.7cm,rmargin=0.7cm}
\usepackage{array}
\usepackage{tabularx}
\setlength{\parindent}{0cm}
\begin{document}
\noindent
\begin{tabularx}{\textwidth}{ |m{12mm}|m{9mm}|m{17mm}|m{11mm}|X|m{18mm}|m{17mm}|r|r|}
  \hline
 \multicolumn{9}{|@{}c@{}|}{\begin{tabularx}{\dimexpr\hsize-2\arrayrulewidth\relax}
                                      {X|X}
50\% & 50\% \\
\hline
50\% & 50\% \end{tabularx}}\\
  \hline
  12mm &
  9mm &
  17mm &
 11mm &
  X &
  18mm &
  17mm &
  r &
  r\\
  \hline
\end{tabularx}
\end{document}