::
per staccare molto puoi usare un codice del genere,
che però non fa quello che vuoi tu
`\documentclass{book}
\usepackage{tocloft,etoolbox,multicol,tikz}
\renewcommand\cftpartafterpnum{\nopagebreak\vspace{-.7em}\par\tikz\draw (0,-0.1) — (0,0) — (\linewidth,0) — (\linewidth, -0.1);\vspace{-2em}}
\setlength{\cftbeforepartskip}{.6em}
\makeatletter
\renewcommand{\cftpartfont}{\bfseries\let\numberline\@gobble}
\renewcommand{\cftpartfillnum}[1]{%
{}%
{\centering\makebox[0em]{\cftpartpagefont}\cftpartafterpnum}%
}
\let\@oldendpart\@endpart
\renewcommand{\@endpart}{\addtocontents{toc}{\protect\begin{multicols}{2}}\@oldendpart}
\makeatother
\let\oldpart\part
\renewcommand{\part}{\ifnum\value{part}>0 \addtocontents{toc}{\protect\end{multicols}}\fi\oldpart}
\AtEndDocument{\ifnum\value{part}>0 \addtocontents{toc}{\protect\end{multicols}}\fi}
\begin{document}
\tableofcontents
\part{Part Title}
\chapter{Chapter Title}
\section{Section Title}
\subsection{Subsection Title}
\subsubsection{Subsubsection Title}
\subsubsection{Subsubsection Title}
\section{Section Title}
\section{Section Title}
\subsection{Subsection Title}
\chapter{Chapter Title}
\section{Section Title}
\subsection{Subsection Title}
\subsubsection{Subsubsection Title}
\subsubsection{Subsubsection Title}
\section{Section Title}
\section{Section Title}
\subsection{Subsection Title}
\part{Part Title}
\chapter{Chapter Title}
\section{Section Title}
\subsection{Subsection Title}
\section{Section Title}
\section{Section Title}
\subsection{Subsection Title}
\subsubsection{Subsubsection Title}
\chapter{Chapter Title}
\section{Section Title}
\subsection{Subsection Title}
\subsubsection{Subsubsection Title}
\subsubsection{Subsubsection Title}
\section{Section Title}
\section{Section Title}
\subsection{Subsection Title}
\part{Part Title}
\chapter{Chapter Title}
\section{Section Title}
\subsection{Subsection Title}
\section{Section Title}
\section{Section Title}
\subsubsection{Subsubsection Title}
\chapter{Chapter Title}
\section{Section Title}
\subsection{Subsection Title}
\subsubsection{Subsubsection Title}
\subsubsection{Subsubsection Title}
\section{Section Title}
\section{Section Title}
\subsection{Subsection Title}
\end{document} `
non ti so dire se usando etoc puoi ottenere il risultato che desideri
m