Draw double line with empty interior in TikZ

By playing around with transparency group one can create the desired effect:

\documentclass[12pt]{article}
\usepackage{tikz}

\begin{document}
\begin{figure}[!htbp]
\centering
\begin{tikzpicture}[scale=4,
    vertex/.style={draw,circle,minimum size=0.75cm,inner sep=0pt},
    arc/.style={draw=blue!50,thick,->},
    arc label/.style={fill=white,font=\tiny,inner sep=1pt},
    loop arc/.style={min distance=2mm}
    ]

\pgfsetblendmode{multiply}    

\begin{scope}[transparency group]    
% Subset 1
\draw[double=none, double distance=60pt, line join=round, line cap=round, draw=black] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\end{scope}

\begin{scope}[transparency group]
% Subset 2
\draw[double=none, double distance=60pt, line join=round, line cap=round, draw=red] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\end{scope}
\end{tikzpicture}
\end{figure}

\end{document}

---

\documentclass[12pt]{article}
\usepackage{tikz}

\begin{document}
\begin{figure}[!htbp]
\centering
\begin{tikzpicture}[scale=4,
    vertex/.style={draw,circle,minimum size=0.75cm,inner sep=0pt},
    arc/.style={draw=blue!50,thick,->},
    arc label/.style={fill=white,font=\tiny,inner sep=1pt},
    loop arc/.style={min distance=2mm}
    ]

\pgfsetblendmode{multiply}

\begin{scope}[transparency group]
\draw[line width=60pt, line join=round, line cap=round, draw=red] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\draw[line width=40pt, line join=round, line cap=round, draw=white] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\draw[line width=60pt, line join=round, line cap=round, draw=black] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\draw[line width=40pt, line join=round, line cap=round, draw=white] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\end{scope}

\begin{scope}[transparency group]
\draw[line width=60pt, line join=round, line cap=round, draw=black] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\draw[line width=40pt, line join=round, line cap=round, draw=white] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\draw[line width=60pt, line join=round, line cap=round, draw=red] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\draw[line width=40pt, line join=round, line cap=round, draw=white] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\end{scope}

\end{tikzpicture}
\end{figure}

\end{document}

This is what I was trying to achieve originally, with a graph on the top layer as follows,

\documentclass[12pt]{article}
\usepackage{tikz}
\usepackage{ifthen}

% Weighted graph from adjacency matrix
\newcommand{\weigthedgraphfromadj}[6][draw,->]{
    \foreach [count=\r] \row in {#3}{
        \foreach [count=\c] \cell in \row{
            \ifnum\cell>0
                \ifnum\c=\r
                    \draw[arc/.try=\cell] (#2\r) edge[loop arc, in= #4*\r -#5 -#4*#6, out= #4*\r +#5 -#4*#6] node[arc label/.try=\cell]{\cell} (#2\c);
                \else
                    \draw[arc/.try=\cell, #1] (#2\r) edge node[arc label/.try=\cell]{\cell} (#2\c);
                \fi
            \fi
        }
    }
}

\begin{document}
\begin{figure}[!htbp]
\centering
\begin{tikzpicture}[scale=4,
    vertex/.style={draw,circle,minimum size=0.75cm,inner sep=0pt},
    arc/.style={draw=blue!50,thick,->},
    arc label/.style={fill=white,font=\tiny,inner sep=1pt},
    loop arc/.style={min distance=2mm}
    ]

\pgfsetblendmode{multiply}    
\begin{scope}[transparency group]    
% Subset 1
\draw[double=none, double distance=60pt, line join=round, line cap=round, draw=black] (1,0)--(0.3090,0.9511)--(-0.8090,0.5878);
\end{scope}

\begin{scope}[transparency group]
% Subset 2
\draw[double=none, double distance=60pt, line join=round, line cap=round, draw=red] (-0.8090,0.5878)--(-0.8090,-0.5878)--(0.3090,-0.9511);
\end{scope}
\pgfsetblendmode{normal}
    \foreach [count=\i] \coord in
                        {(0.3090,0.9511),
                        (-0.8090,0.5878),
                        (-0.8090,-0.5878),
                        (0.3090,-0.9511),
                        (1,0)}
                        {
        \ifthenelse{\i<4}{\node[vertex] (p\i) at \coord {$x_{\i}$}}{\ifthenelse{\i=4}{\node[vertex] (p\i) at \coord {$u_{2}$}}{\node[vertex] (p\i) at \coord {$u_{1}$}}};
    }
    \weigthedgraphfromadj[bend left=10]{p}{{1,1,1,0,0},
                                            {24,20,24,0,0},
                                            {1,1,1,0,0},
                                            {0,0,2,0,0},
                                            {2,0,0,0,0}}{72}{30}{0}
\end{tikzpicture}
\end{figure}

\end{document}

There is a graph macro plotting graphs from some coordinates and an adjacency matrix. The sets are overlapping weighted directed subgraphs.

Hope it can be of some use for others.

Romain