pgf-plot mit zwei Achsen

 %\RequirePackage{filecontents}
\begin{filecontents}{\jobname B.dat}

25.0   17.3
27.4   17.9
29.7   19.1
32.1   20.2
34.5   21.3
36.9   22.7
39.2   23.8
41.6   24.8
44.0   26.2
46.4   27.2
48.7   28.2
51.1   29.3
53.5   30.5
55.9   31.6
58.2   32.9
60.6   34.2
63.0   35.1
65.4   36.4
67.7   37.5
70.1   38.5
72.5   39.6
74.9   40.8
77.2   42
79.6   43.3
82.0   44.5
84.4   45.5
86.7   46.5
89.1   47.6
91.5   48.8
93.9   49.9
96.2   51.1
98.6   52.2 
\end{filecontents}

\begin{filecontents}{\jobname A.dat}
25.0   691
27.4   654
29.7   641
32.1   629
34.5   619
36.9   615
39.2   606
41.6   596
44.0   594
46.4   585
48.7   579
51.1   574
53.5   570
55.9   566
58.2   564
60.6   563
63.0   557
65.4   556
67.7   554
70.1   549
72.5   547
74.9   545
77.2   544
79.6   544
82.0   542
84.4   539
86.7   536
89.1   535
91.5   533
93.9   532
96.2   531
98.6   529 
\end{filecontents}

\documentclass{scrbook}
\usepackage{pgfplots}                       
\usepackage{siunitx}                       
\usepgfplotslibrary{units}
\pgfplotsset{
    width=\textwidth,
    height = 0.4\textheight,
    compat=newest,
    minor tick style={black},
    legend style={cells={anchor=west}},
    every axis legend/.append style={at={(0.99,
    0.99)},
    anchor=northeast},
    ticklabel shift={0.05cm}
 }

\begin{document}

\begin{figure}[htb]
\centering
\begin{tikzpicture}
\begin{axis}[grid = major, width = \textwidth-45pt, height = 0.4\textheight,% max space between ticks=50,
            xlabel near ticks,
            ylabel near ticks,
            xlabel = {Scherrate},
            x unit=\si{\second},
            ylabel = {aaa},
            y unit=\si{\percent},
            xmin=0, xmax=100,
            ymin=400, ymax=700,
            x tick label style={/pgf/number format/1000 sep=}
            ]
    \addplot[color = blue, mark = none] table {\jobname A.dat};
    \label{Viskosität}
\end{axis}

\begin{axis}[ylabel near ticks, width = \textwidth-45pt, height = 0.4\textheight,
            axis y line*=right,
            ylabel = {xxx},
            y unit=\si{\percent},
            xmin=0, xmax=100,
            ymin=0, ymax=300,
            x tick label style={/pgf/number format/1000 sep=},
            legend pos=north east,
            legend cell align=left,
            legend style={at={(0.99,0.99)}}
            ]
    \addplot[color = red, mark = none] table {\jobname B.dat};
    \addlegendimage{/pgfplots/refstyle=Viskosität}
    \addlegendentry{Schubspannung}
    \addlegendentry{Viskosität}
\end{axis}
\end{tikzpicture}
\end{figure}

\end{document}