Updated Version of -X>

\documentclass[border=1mm,tikz]{standalone}
\usepackage{chemfig}
\usepackage[version=4,arrows=pgf-filled]{mhchem}

% allow for editing Chemfig
\catcode`\_=11

% Initial arguments:
% #1, #2: Same as for -U> (above arrow)
% #3: Additional label at midpoint (also above arrow)
% #4, #5, #6: Like #1, #2, and #3, but below arrow
% #7: Optional shift, default 0
% #8: Optional arrow radius
% #9: Optional arrow angle
\definearrow9{-X>}{%
    \CF_arrowshiftnodes{#7}%
    \expandafter\draw\expandafter[\CF_arrowcurrentstyle](\CF_arrowstartnode)--(\CF_arrowendnode)node[midway](Xarrowarctangent){};%
    \CF_ifempty{#8}
        {\def\CF_Xarrowradius{0.333}}
        {\def\CF_Xarrowradius{#8}}%
    \CF_ifempty{#9}%
        {\def\CF_Xarrowabsangle{60}}
        {\pgfmathsetmacro\CF_Xarrowabsangle{abs(#9)}}
    % Draw top arrow (start)
    \edef\CF_tmpstr{[\CF_ifempty{#1}{draw=none}{\unexpanded\expandafter{\CF_arrowcurrentstyle}},-]}%
    \expandafter\draw\CF_tmpstr (Xarrowarctangent)%
    arc[radius=\CF_compoundsep*\CF_currentarrowlength*\CF_Xarrowradius,start angle=\CF_arrowcurrentangle-90,delta angle=-\CF_Xarrowabsangle]node(Xarrow1start){};
    % Draw bottom arrow (end)
    \edef\CF_tmpstr{[\CF_ifempty{#2}{draw=none}{\unexpanded\expandafter{\CF_arrowcurrentstyle}},-CF]}%
    \expandafter\draw\CF_tmpstr (Xarrowarctangent)%
    arc[radius=\CF_compoundsep*\CF_currentarrowlength*\CF_Xarrowradius,%
    start angle=\CF_arrowcurrentangle-90,%
    delta angle=\CF_Xarrowabsangle]%
    node(Xarrow1end){};
    % Draw bottom arrow (start)
    \edef\CF_tmpstr{[\CF_ifempty{#4}{draw=none}{\unexpanded\expandafter{\CF_arrowcurrentstyle}},-]}%
    \expandafter\draw\CF_tmpstr (Xarrowarctangent)%
    arc[radius=\CF_compoundsep*\CF_currentarrowlength*\CF_Xarrowradius,start angle=\CF_arrowcurrentangle+90,delta angle=\CF_Xarrowabsangle]node(Xarrow2start){};
    % Draw bottom arrow (end)
    \edef\CF_tmpstr{[\CF_ifempty{#5}{draw=none}{\unexpanded\expandafter{\CF_arrowcurrentstyle}},-CF]}%
    \expandafter\draw\CF_tmpstr (Xarrowarctangent)%
    arc[radius=\CF_compoundsep*\CF_currentarrowlength*\CF_Xarrowradius,%
    start angle=\CF_arrowcurrentangle+90,%
    delta angle=-\CF_Xarrowabsangle]%
    node(Xarrow2end){};
    % Insert labels
    \pgfmathsetmacro\CF_tmpstra{\CF_Xarrowradius*cos(\CF_arrowcurrentangle)<0?"-":"+"}%
    \pgfmathsetmacro\CF_tmpstrb{\CF_Xarrowradius*cos(\CF_arrowcurrentangle)<0?"+":"-"}%
    \ifdim\CF_Xarrowradius pt>0 pt
        \CF_arrowdisplaylabel{#1}{0}\CF_tmpstra{Xarrow1start}{#2}{1}\CF_tmpstra{Xarrow1end}%
        \CF_arrowdisplaylabel{#4}{0}\CF_tmpstrb{Xarrow2start}{#5}{1}\CF_tmpstrb{Xarrow2end}%
        \CF_arrowdisplaylabel{#3}{0.5}\CF_tmpstra\CF_arrowstartnode{}{}{}\CF_arrowendnode%
        \CF_arrowdisplaylabel{#6}{0.5}\CF_tmpstrb\CF_arrowstartnode{}{}{}\CF_arrowendnode%
    \else
        \CF_arrowdisplaylabel{#2}{0}\CF_tmpstra{Xarrow1start}{#1}{1}\CF_tmpstra{Xarrow1end}%
        \CF_arrowdisplaylabel{#5}{0}\CF_tmpstrb{Xarrow2start}{#4}{1}\CF_tmpstrb{Xarrow2end}%
        \CF_arrowdisplaylabel{#3}{0.5}\CF_tmpstra\CF_arrowstartnode{}{}{}\CF_arrowendnode%
        \CF_arrowdisplaylabel{#6}{0.5}\CF_tmpstrb\CF_arrowstartnode{}{}{}\CF_arrowendnode%
    \fi
}
% stop editting chemfig
\catcode`\_=8


\begin{document}
\schemestart
A
\arrow{-X>[1][2][3][4][5][6]}
B
\schemestop
\end{document}