two links between nodes in tikzpicture

Question

I'd like to draw two links between nodes with directions of arrowheads being opposite. Please see the picture below.

I tried something like:

\draw[->] ([yshift=0.1cm]1.east) -- ([yshift=0.1cm]2.west);
\draw[<-] ([yshift=-0.1cm]1.east) -- ([yshift=-0.1cm]2.west);

It works, but every time I draw, I have to know it is east, west, north, or south. Also, I need to write two lines for each pair.

Is there anyway like

\draw[somestyle] (1) -- (2);;

that works for not only horizontal links, but also vertical and diagonal links?

Answer 1

This defines a style of this sort. It is based on this answer, and really works in this for circular nodes only.

\documentclass[tikz,border=3.14mm]{standalone}
\usetikzlibrary{positioning,quotes}
\usetikzlibrary{calc}
\tikzset{shifted path/.style args={from #1 to #2 with label #3}{insert path={
let \p1=($(#1.east)-(#1.center)$),
\p2=($(#2.east)-(#2.center)$),\p3=($(#1.center)-(#2.center)$),
\n1={veclen(\x1,\y1)},\n2={veclen(\x2,\y2)},\n3={atan2(\y3,\x3)} in
(#1.{\n3+180+asin(\pgfkeysvalueof{/tikz/shifted path/dist}/\n1)}) edge[/tikz/shifted path/arrows,"#3"] (#2.{\n3-asin(\pgfkeysvalueof{/tikz/shifted path/dist}/\n2)})
}},back and forth/.style={/utils/exec=\pgfkeys{/tikz/shifted path/.cd,#1},
shifted path=from \pgfkeysvalueof{/tikz/shifted path/from} to \pgfkeysvalueof{/tikz/shifted path/to} with label \pgfkeysvalueof{/tikz/shifted path/label 1},
shifted path=from \pgfkeysvalueof{/tikz/shifted path/to} to \pgfkeysvalueof{/tikz/shifted path/from} with label \pgfkeysvalueof{/tikz/shifted path/label 2}},
shifted path/.cd,dist/.initial=3pt,arrows/.style={-stealth},from/.initial=1,to/.initial=2,label 1/.initial={},label 2/.initial={}}
\begin{document}

\begin{tikzpicture}
   \begin{scope}[every node/.style={draw, circle}]
    \path (0,2) node(1){1} (6,2) node(2){2}
     (0,0) node(3){3} (2,0) node(4){4}   (4,0) node(5){6} (6,0) node(6){6};
   \end{scope}
   \begin{scope}[]
   \draw[shifted path/arrows/.style={stealth-},
   back and forth/.list={{from=1,to=2,label 1=3,label 2=1},
    {from=1,to=3,label 1=2,label 2=5},
    {from=2,to=6,label 1=4,label 2=14},
    {from=3,to=4,label 1=8,label 2=12},
    {from=4,to=5,label 1=7,label 2=15},
    {from=5,to=6,label 1=11,label 2=17}}];
   \end{scope}  
\end{tikzpicture}
\end{document}

Please note that this very diagram can be conveniently produced with tikz-cd.