No, you don't have to use conditionals. paracol
has the option/command \twosided
for that, which, according to what is written in the manual in p. 18 supersedes \swapcolumninevenpages
(which is how I found this command, i.e. even though \swapcolumninevenpages
is lengthy it describes more clearly what this does). This is illustrated in the following MWE where the Y column is left/right depending on whether the page number is even/odd.
\documentclass[12pt,a4paper]{book}
\usepackage{paracol}
\newcounter{dummy}
\newcommand{\dummycolumn}[2]{\setcounter{dummy}{0}%
\loop
#1\stepcounter{dummy}\par
\ifnum\number\value{dummy}<#2
\repeat}
\begin{document}
\twosided
\begin{paracol}{2}
\dummycolumn{X}{200}
\switchcolumn
\dummycolumn{Y}{200}
\end{paracol}
\end{document}
