There appear to be two missing anchor node names when using shapes.multipart rectangle split horizontal, indicated in the code below. It may be that they are called something else but I could not find it in pgflibraryshapes.multipart.code.tex.
\documentclass[border=0.2in]{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes.multipart}
\begin{document}
\begin{tikzpicture}
\node[%
rectangle split,
rectangle split horizontal,
rectangle split parts=2,
draw=lightgray
] (mybox) {
\parbox[t][0.5in]{1in}{Box 1}
\nodepart{two}\parbox[t][0.5in]{2in}{Box 2}
};
%% Locating the missing anchors:
\coordinate (missingsouth) at (mybox.two split south -| mybox.text split);
\coordinate (missingnorth) at (mybox.two split north -| mybox.text split);
\node[red] at (missingsouth) {S}; %% <nodname>.text split south gives TikZ error
\node[blue] at (missingnorth) {N}; %% <nodname>.text split north gives TikZ error
\end{tikzpicture}
\end{document}
Please ignore spurious code
</nodname></nodname>after\end{tikzpicture}. I don't know how that got there...These anchors are not documented. If you need them, feel free to add them and submit a merge request.