Two coloured arrow

A very basic suggestion to start with:

\begin{filecontents*}{twocoloredline.asy}
import graph;

struct DblLine{
  typedef pair T_fParam(real t);

  guide gline,midLine,leftLine,rightLine;
  arrowhead arrowHeadStyle;
  guide arhd;  // arrow head outline;
  guide leftAr, rightAr;
  pen leftPen, rightPen;

  T_fParam fmid(guide g){return new pair(real t){return point(g,t);} ;}

  T_fParam fleft(guide g,pen linepen=currentpen, real sc=linewidth(linepen)){
    return new pair(real t){
      pair m=point(g,t);
      pair p=m+rotate(90)*scale(sc)*dir(g,t);
      return p;
    };
  }

  T_fParam fright(guide g,pen linepen=currentpen, real sc=linewidth(linepen)){
    return new pair(real t){
      pair m=point(g,t);
      pair p=m+rotate(-90)*scale(sc)*dir(g,t);
      return p;
    };
  }

  void prepareHead(){
    arhd=arrowHeadStyle.head(midLine,leftPen);
    int midPointNo=ceil(size(arhd)/2);
    leftAr=subpath(arhd,0,midPointNo)--cycle;
    rightAr=subpath(arhd,midPointNo,size(arhd))--cycle;
  }  

  void prepareLine(){
    midLine=graph(fmid(gline),0.0,(real)size(gline),operator..);
    leftLine=graph(fleft(gline,leftPen),0.0,(real)size(gline),operator..);
    rightLine=graph(fright(gline,rightPen),0.0,(real)size(gline),operator..);

    if(arrowHeadStyle!=null){
      prepareHead();  
      leftLine=subpath(leftLine,0.0,intersect(leftLine,arhd)[0]);
      rightLine=subpath(rightLine,0.0,intersect(rightLine,arhd)[0]);
    }
  }

  void draw(){
    fill(midLine--reverse(leftLine)--cycle,leftPen);
    fill(rightLine--reverse(midLine)--cycle,rightPen);
    if(arrowHeadStyle!=null){
      fill(leftAr,leftPen);
      fill(rightAr,rightPen);
    }
  }

  void operator init(guide gline, arrowhead arrowHeadStyle=null,pen leftPen=red,pen rightPen=blue){
    this.gline=gline;
    this.arrowHeadStyle=arrowHeadStyle;
    this.leftPen=leftPen;
    this.rightPen=rightPen;
    prepareLine();
  }
}

////// Example of two-colored-line drawings:
//
//  import twocoloredline;
//  size(100);
//  guide g=scale(0.5)*rotate(60)*shift(-100,-50)*((0,0)::(100,100){up}::(0,200){down});
//  DblLine dblg=DblLine(shift(-20,-20)*scale(0.5)*g,blue,red);       // blue/red line
//  dblg.draw();
//  
//  DblLine(shift(-10,-5)*g, HookHead).draw();
//  
//  DblLine(shift(-100,-50)*((10,0)--(50,0)), HookHead,green+.4pt,blue).draw();
//  
//  DblLine(shift(-100,-50)*((10,0)--(0,50)), HookHead,red+0.2pt,green).draw();
//  
//  
//  DblLine((-40,-40)..(0,-40){up}..{up}(0,-10)..(-40,-10), HookHead,green,lightred+0.2pt).draw();

\end{filecontents*}
\documentclass{article}
\usepackage{asymptote}
\begin{document}

\begin{asy}
// Example of two-colored-line drawings:

import twocoloredline;
size(100);
guide g=scale(0.5)*rotate(60)*shift(-100,-50)*((0,0)::(100,100){up}::(0,200){down});
DblLine dblg=DblLine(shift(-20,-20)*scale(0.5)*g,blue,red);       // blue/red line
dblg.draw();
DblLine(shift(-10,-5)*g, HookHead).draw();
DblLine(shift(-100,-50)*((10,0)--(50,0)), HookHead,green+.4pt,blue).draw();
DblLine(shift(-100,-50)*((10,0)--(0,50)), HookHead,red+0.2pt,green).draw();
DblLine((-40,-40)..(0,-40){up}..{up}(0,-10)..(-40,-10), HookHead,green,lightred+0.2pt).draw();
\end{asy}

\end{document}

\documentclass{article}
\usepackage{xcolor}
\usepackage{verbatimbox}
\begin{document}
\shortstack{\addvbuffer[0ex -.72ex]{$\textcolor{red}{\leftharpoonup}$}\\
                \addvbuffer[-.72ex 0ex]{$\textcolor{blue}{\leftharpoondown}$}}

\shortstack{\addvbuffer[0ex -.72ex]{$\textcolor{red}{\rightharpoonup}$}\\
                \addvbuffer[-.72ex 0ex]{$\textcolor{blue}{\rightharpoondown}$}}
\end{document}