documentclass{article}

    % UNITS
usepackage{siunitx}
sisetup{per=slash, load=abbr}

    % GRAPHICS
usepackage{tikz}
usepackage{pgfplots}
pgfplotsset{width=7cm,compat=1.3}

begin{document}

begin{tikzpicture}
pgfplotsset{
    scale only axis,
    scaled x ticks=base 10:3,
    xmin=0, xmax=0.06
}

begin{axis}[
  axis y line*=left,
  ymin=0, ymax=80,
  xlabel=x-axis,
  ylabel=y-axis 1,
]
addplot[smooth,mark=x,red]
  coordinates{
    (0,68.6)
    (0.0148,72)
    (0.0295,68.6)
    (0.0441,53.4)
    (0.059,22.8)
}; label{plot_one}
%addlegendimage{/pgfplots/refstyle=Hplot}addlegendentry{plot 1}
addplot[smooth,mark=o,green]
  coordinates{
    (0,60)
    (0.0148,65)
    (0.0295,73)
    (0.0441,49)
    (0.059,19)
}; label{plot_two}
%addlegendentry{plot 2}
end{axis}

begin{axis}[
  axis y line*=right,
  axis x line=none,
  ymin=0, ymax=100,
  ylabel=y-axis 2
]
addlegendimage{/pgfplots/refstyle=plot_one}addlegendentry{plot 1}
addplot[smooth,mark=*,blue]
  coordinates{
    (0,0)
    (0.0148,48)
    (0.0295,66)
    (0.0441,66)
    (0.059,45.0)
}; addlegendentry{plot 3}
end{axis}

end{tikzpicture}
end{document}