Reduced Planck's constant with two crossings

You can add a lowered bar to \hbar.

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand{\hhbar}{%
  \text{\m@th
    \makebox[0pt][l]{\raisebox{-0.15\height}{$\mathchar'26$}}%
  }\hbar
}
\makeatother

\begin{document}

$\hbar\hhbar\scriptstyle\hbar\hhbar\scriptscriptstyle\hbar\hhbar$

$xhx$

$x\hbar x$

$x\hhbar x$

\end{document}

Like this?

\documentclass{article}
\newcommand{\hbbar}{{\raisebox{0.05ex}{$\mathchar '26$}\mkern -9mu\raisebox{-0.15ex}{$\mathchar '26$}\mkern -9muh}}
\begin{document}
$p=\hbbar k$

$p=\hbar k$

$p=hk$

\end{document}

It is true that this does not transform nicely under \scriptstyle. The following makes use of egreg's answer to fix this, but keeps the bars more evenly distributed as above.

\documentclass{article}
\usepackage{amsmath}
\makeatletter
\newcommand{\hbbar}{%
  \text{\m@th
    \makebox[0pt][l]{\raisebox{0.05\height}{$\mathchar'26$}}%
    \makebox[0pt][l]{\raisebox{-0.12\height}{$\mathchar'26$}}%
  }h
}
\makeatother

\begin{document}
$p=\hbbar k$ $\scriptstyle p=\hbbar k$ $\scriptscriptstyle p=\hbbar k$

$p=\hbar k$ $\scriptstyle p=\hbar k$ $\scriptscriptstyle p=\hbar k$

$p=hk$ $\scriptstyle p=h k$ $\scriptscriptstyle p=h k$

\end{document}