How to show \listoffigures and \listoftables on one page and in the ToC?

You can temporally disable the \clearpage (and/or \cleardoublepage) command which generates the page break at the beginning of the second \listof... command:

\listoffigures
\begingroup
\let\clearpage\relax
\listoftables
\endgroup

You might need to manually adjust the spacing before the second headline.

However without a minimal example that shows your used class and packages I can't test it. It works with the default book class, but then the LoF and LoT aren't added to the ToC in any case.

Just an afterthough. For documents with few tables and figures I usualy bundle them together in one command \listofillustrations

\makeatletter
\providecommand\phantomsection{}% for hyperref

\newcommand\listofillustrations{%
    \chapter*{List of Illustrations}%
    \phantomsection
    \addcontentsline{toc}{chapter}{List of Illustrations}%
    \section*{Figures}%
    \phantomsection
    \addcontentsline{toc}{section}{\protect\numberline{}Figures}%
    \@starttoc{lof}%
    \bigskip
    \section*{Tables}%
    \phantomsection
    \addcontentsline{toc}{section}{\protect\numberline{}Tables}%
    \@starttoc{lot}}

\makeatother

If you are using an article class you can use \section* and \subsection*

memoir's default \listoffigures and \listoftables commands don't put themselves on separate pages:

\documentclass{memoir}

\begin{document}
\tableofcontents*
\listoftables
\listoffigures

\chapter{Introduction}
\begin{figure}
\centering
Some fake content
\caption{A first fake figure}%
\end{figure}

\begin{figure}
\centering
Some fake content
\caption{A second fake figure}%
\end{figure}

\begin{table}
\centering
\begin{tabular}{lcr}
More & fake & content
\end{tabular}
\caption{A first fake table}
\end{table}

\begin{table}
\centering
\begin{tabular}{lcr}
More & fake & content
\end{tabular}
\caption{A second fake table}
\end{table}
\end{document}

I won't go so far as to recommend memoir for everything, but if you're writing a thesis, and are already looking at tocloft and similar packages, you may end up piecing together a sizable portion of what memoir already provides in one class. I have a style for my university based on memoir, and the lines of code required to maintain the style requirements is easily half of what the previous style file had. How much of that improvement is due to memoir itself, and how much is from replacing old explicit code with newer CTAN packages, I can't say.