No \xrightrightarrow[a]{b}?

The following example defines \xtwoheadrightarrow and \xtwoheadleftarrow similar to the definitions of \xrightarrow and \xleftarrow:

\documentclass{article}
\usepackage{amssymb}
\usepackage{amsmath}

\makeatletter
\providecommand*{\twoheadrightarrowfill@}{%
  \arrowfill@\relbar\relbar\twoheadrightarrow
}
\providecommand*{\twoheadleftarrowfill@}{%
  \arrowfill@\twoheadleftarrow\relbar\relbar
}
\providecommand*{\xtwoheadrightarrow}[2][]{%
  \ext@arrow 0579\twoheadrightarrowfill@{#1}{#2}%
}
\providecommand*{\xtwoheadleftarrow}[2][]{%
  \ext@arrow 5097\twoheadleftarrowfill@{#1}{#2}%
}
\makeatother

\begin{document}
\[
  A \xrightarrow[under]{over} B
  \xtwoheadrightarrow[under]{over} C
  \xtwoheadleftarrow[under]{over} D
\]
\end{document}

The case \rightrightarrows/\leftleftarrows is much more complicate. Before there is a symbol \relbar that is used for the extensible part of the arrow. But there is no symbol for the double line case. Thus the vertical positions of the lines and their thickness is not known. The following example defines \relrelbar for this purpose that is composed of \relbar lowered and raised by the amount given in macro \relrelbarsep. Therefore this macro needs to be redefined for a different font. The example uses the AMS fonts:

\documentclass{article}
\usepackage{amssymb}
\usepackage{amsmath}

\makeatletter
\newcommand*{\relrelbarsep}{.386ex}
\newcommand*{\relrelbar}{%
  \mathrel{%
    \mathpalette\@relrelbar\relrelbarsep
  }%
}
\newcommand*{\@relrelbar}[2]{%
  \raise#2\hbox to 0pt{$\m@th#1\relbar$\hss}%
  \lower#2\hbox{$\m@th#1\relbar$}%
}
\providecommand*{\rightrightarrowsfill@}{%
  \arrowfill@\relrelbar\relrelbar\rightrightarrows
}
\providecommand*{\leftleftarrowsfill@}{%
  \arrowfill@\leftleftarrows\relrelbar\relrelbar
}
\providecommand*{\xrightrightarrows}[2][]{%
  \ext@arrow 0359\rightrightarrowsfill@{#1}{#2}%
}
\providecommand*{\xleftleftarrows}[2][]{%
  \ext@arrow 3095\leftleftarrowsfill@{#1}{#2}%
}
\makeatother

\begin{document}
\[
  A \xrightrightarrows[under]{over} B
  \xleftleftarrows[under]{over} C
\]
\end{document}

The numbers 0579, ... are four arguments for \ext@arrow that is defined by package amsmath:

\def\ext@arrow#1#2#3#4#5#6#7{%
  \mathrel{\mathop{%
    \setbox\z@\hbox{#5\displaystyle}%
    \setbox\tw@\vbox{\m@th
      \hbox{$\scriptstyle\mkern#3mu{#6}\mkern#4mu$}%
      \hbox{$\scriptstyle\mkern#3mu{#7}\mkern#4mu$}%
      \copy\z@
    }%
    \hbox to\wd\tw@{\unhbox\z@}}%
  \limits
    \@ifnotempty{#7}{^{\if0#1\else\mkern#1mu\fi
                       #7\if0#2\else\mkern#2mu\fi}}%
    \@ifnotempty{#6}{_{\if0#1\else\mkern#1mu\fi
                       #6\if0#2\else\mkern#2mu\fi}}}%
}

Some remarks:

• The width of box \tw@ contains the maximal width of the text below. above and the symbol without text.
• The latter two numbers specify the margins for the texts in the measurement phase.
• The first two numbers specify the margins for the texts that is finally set and allows to insert some asymmetry of the horizontal placement because of the arrows.

Adapting an answer from How do I put text over a squiggly arrow?. See that answer for a full description of the created macros, though, in the end, the commands to use are \rextlinearrow{string}{integer} for a right arrow and \lextlinearrow{string}{integer} for a left arrow. The integer is the arrow-shaft length defined in terms of multiples of a minus sign. The string is the text to be written over the arrow.

\documentclass[12pt]{article}
\parskip 1ex
\usepackage{amssymb}
\usepackage{ifthen}
\textwidth 2in
\begin{document}
Here are the commands to make extensible arrows, with no text above
them:

\newcommand{\extline}{$\scriptsize$-$\normalsize$\!}
\newcommand{\lextlineend}{$\scriptsize$\lhd\!$\normalsize$}
\newcommand{\rextlineend}{$\scriptsize\rule{.1ex}{0ex}$\rhd$\normalsize$}
\noindent
$2H_2 + O_2 ~\extline\extline\extline\rextlineend~ 2H_2O $\\
$2H_2 + O_2 ~\lextlineend\extline\extline\extline~ 2H_2O $

Now I will create commands to place text over them

\newcounter{index}

\newcommand\extlines[1]{%
  \setcounter{index}{0}%
  \whiledo {\value{index}< #1}
  {\addtocounter{index}{1}\extline}
}

\newcommand\rextlinearrow[2]{$
  \setbox0\hbox{$\extlines{#2}\rextlineend$}%
  \tiny$%
  \!\!\!\!\begin{array}{c}%
  \mathrm{#1}\\%
  \usebox0%
  \end{array}%
  $\normalsize$\!\!%
}

\newcommand\lextlinearrow[2]{$
  \setbox0\hbox{$\lextlineend\extlines{#2}$}%
  \tiny$%
  \!\!\!\!\begin{array}{c}%
  \mathrm{#1}\\%
  \usebox0%
  \end{array}%
  $\normalsize$\!\!%
}

\noindent
$2H_2 + O_2 \rextlinearrow{exothermic}{7} 2H_2O $\\
$2H_2 + O_2 \lextlinearrow{heat}{3} ~ 2H_2O $

\end{document}