\begin{center}

\input{plp}
\input{plp2}

\begin{center}
\begin{tikzpicture}%
[>=stealth, help lines/.style={very thin,draw=black!25}, x=1cm, y=1cm]

% draw grid of help lines
\draw[help lines] (0,0) grid (12,4);

% place both structures
\node[anchor=south west, inner sep=10pt] at (0,0) (plp) {\chemfig{!{plp}}};
\node[anchor=south west, inner sep=10pt] at (7,0) (plp2) {\chemfig{!{plp2}}};

\draw[<->] (plp) -- (plp2);

\end{tikzpicture}
\end{center}