How to merge two harpoon arrows together?

Question

I want to merge a \leftharpoondown and a \rightharpoonup together perfectly, no text necessary. Left/right harpoon arrow with text? provided some solutions, but each arrowhead has some of the arrowtail of the other arrow jutting out, unfortunately (because the head tapers while the tail has thickness).

Answer 1

Here's how to borrow these symbols from mathabx without changing all your symbols:

\documentclass[11pt]{article}
\DeclareFontFamily{U}{mathb}{\hyphenchar\font45}
\DeclareFontShape{U}{mathb}{m}{n}{%
<-6> matha5
<6-7> matha6
<7-8> mathb7
<8-9> mathb8
<9-10> mathb9
<10-12> mathb10
<12-> mathb12
}{}%
\DeclareSymbolFont{mathb}{U}{mathb}{m}{n}%
\DeclareMathSymbol{\leftrightharpoon}{\mathrel}{mathb}{"E0}
\DeclareMathSymbol{\rightleftharpoon}{\mathrel}{mathb}{"E1}
\begin{document}
[ a \leftrightharpoon b \rightleftharpoon c ]%
\end{document}