\documentclass[11pt,letterpaper,openany]{scrbook}
\usepackage[letterpaper, bindingoffset=0.2in, left=1in,right=1in,top=.5in,bottom=.5in,footskip=.25in,marginparwidth=5em]{geometry}

\usepackage{marginnote}
\usepackage{sectsty}
\usepackage{ragged2e}
\usepackage{paracol}
\usepackage{fontspec}

\allsectionsfont{\centering}

\setlength\parindent{0pt}
\setlength{\columnsep}{1.25em}
\setlength{\parfillskip}{0pt}
\setlength{\tabcolsep}{1em}

\raggedbottom

\begin{document}
\columnratio{0.5}\begin{paracol}{1}
hello world
\end{paracol}
\end{document}