\begin{tikzpicture}[node distance=1.2cm,>=stealth',bend
  angle=25,auto,transform shape]

  \tikzstyle{place}=[circle,thick,draw=red!15,fill=red!5,minimum size=5mm]
  \tikzstyle{transition}=[rectangle,thick,draw=blue!15,fill=blue!5,minimum size=4mm]

  \tikzstyle{edge}=[black!25!black!25]
  \tikzstyle{every label}=[black]

  \begin{scope}
    % Petri net A
    \node [place] (p1) [tokens=0] [xshift=-3.5cm]{}
    ;

    \node [transition] (t1) [below of=p1] {}
      edge [pre,black!25]  (p1)
    ;

    %%in order to center tstart transition
    \node [place] (p) [below of=t1,circle,draw=white,fill=white] {};

    \node [place] (p2) [left of=p] {}
      edge [pre,black!25] (t1)
    ;

    \node [place] (p3)  [right of=p,draw=red!75,fill=red!20,dash pattern=on 2pt off 2pt] {}
      edge [pre,black!25] (t1)
    ;
%    \node at (p3.west) [left] {{$start$}};

    \node [transition] (t2) [below of=p2] {}
      edge [pre,black!25]  (p2)
    ;

    \node [place] (p4) [below of=t2] {}
      edge [pre,black!25] (t2)
    ;

    % Petri net B
    \node [place] (pb1) [tokens=0] {}
    ;

    %dotted,
    \node [transition] (tb1) [below of=pb1] {}
      edge [pre,black!25] (pb1)
      ;

    %%in order to center tstart transition
    \node [place] (pp) [below of=tb1,circle,draw=white,fill=white] {};
    \node [place] (pb2) [left of=pp] {}
      edge [pre,black!25] (tb1)
    ;
    \node [place] (pb3) [right of=pp] {}
      edge [pre,black!25] (tb1)
    ;

    \node [transition] (tb2) [below of=pb2,draw=blue!75,fill=blue!20,dash pattern=on 2pt off 2pt] {}
      edge [pre,black!25] (pb2)
      edge [pre,bend right,green!50!black] (p3)
      ;
%    \node at (tb2.east) [right] {{$finish$}};

    \node [place] (pb4) [below of=tb2] {}
      edge [pre,black!25] (tb2)
    ;

  \end{scope}

\end{tikzpicture}