begin{tikzpicture}
    defL{1.5}
    draw (0,0) to[bend left] (L,0) node[pos=0.5] (a) {};
    draw (L,0) to[bend left] (0,0) node[pos=0.5] (b) {};
    draw[dashed] (a) -- (b);
    fill[red] (a) circle(1.5pt);
    fill[blue] (a) circle(1.5pt);
end{tikzpicture}