\documentclass[varwidth=true, border=2pt]{standalone}
\usepackage[utf8]{inputenc} % this is needed for umlauts
\usepackage[ngerman]{babel} % this is needed for umlauts
\usepackage[T1]{fontenc}    % this is needed for correct output of umlauts in pdf
\usepackage[margin=2.5cm]{geometry} %layout

\usepackage{pgfplots}

\begin{document}
\begin{tikzpicture}
    \begin{axis}[
            axis x line=middle,
            axis y line=middle,
            enlarge y limits=true,
            xmin=0, xmax=2150,
            ymin=0, ymax=20000000,
            width=15cm, height=8cm,     % size of the image
            grid = major,
            grid style={dashed, gray!30},
            ylabel=Stored game situations,
            xlabel=seconds,
            legend style={at={(0.1,-0.1)}, anchor=north}
         ]
          \addplot table [x=seconds, y=situations, col sep=comma] {linearProbing.csv};
          \addplot table [x=seconds, y=situations, col sep=comma] {quadraticProbing.csv};
          \legend{linear probing,quadratic probing}
    \end{axis}

    % this is the right axis
    \begin{axis}[
        hide x axis,
        axis y line*=right,
        width=15cm, height=8cm,     % size of the image
    ]
          \addplot[orange, mark=halfcircle] table [x=seconds, y=mirrored, col sep=comma] {linearProbing.csv};
          \addplot[black, mark=x] table [x=seconds, y=mirrored, col sep=comma] {quadraticProbing.csv};
    \end{axis}
\end{tikzpicture}
\end{document}