Curved double harpoon (equilibrium) arrows in Chemfig

I did it! It's not ideal, but I did it. it is a benzene formation scheme.

\documentclass[a4paper]{article}

\usepackage[T2A]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english, russian]{babel}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage[colorinlistoftodos]{todonotes}
\usepackage{chemfig}


\begin{document}


\begin{figure}[h!]
    \tiny
    \begin{center}
        \noindent
        \makeatletter
        \definearrow1{s>}{%
            \ifx\@empty#1\@empty
            \expandafter\draw\expandafter[\CF@arrow@current@style,-CF](\CF@arrow@start@node)--(\CF@arrow@end@node);%
            \else
            \def\curvedarrow@style{shorten <=\CF@arrow@offset,shorten >=\CF@arrow@offset,}%
            \CF@expadd@tocs\curvedarrow@style\CF@arrow@current@style
            \expandafter\draw\expandafter[\curvedarrow@style,-CF](\CF@arrow@start@name)..controls#1..(\CF@arrow@end@name);
            \fi
        }
        \makeatother
        \schemedebug{false}
        \schemestart
        \noindent
        \setcrambond{2pt}{}{}
        \chemfig{ % Benzene Center
            [:60]**6(------)} % Команда [:60] обозначает поворот молекулы на 60 град. Для ЦГ это незаметно. Однако baseline теперь внизу молекулы.
        \arrow(benz.north--ch.south){<=>[][][]}[90,2] % Линия к циклогексану
        \chemfig{(*6(------))} % циклогексан
        \arrow(@benz.125--mch.305){<=>[][][]}[125,2,,,draw=none] % Линия к метилциклогексану
        \chemfig{(*6(----(-)--))} % метилциклогексан
        \arrow(@benz.192--tol.east){<=>[][][]}[185,2,,,draw=none] % Линия к толуолу
        \chemfig{(**6(-----(-)-))} % толуол
        \arrow(@benz.226--xy.10){<=>[][][]}[219,2,,,draw=none] % Линия к ксилолу
        \chemfig{(**6((-)----(-)--))} % ксилол
        \arrow(@benz.255--eb.45){<=>[][][]}[248,2,,,draw=none] % Линия к этилбензолу
        \chemfig{(**6((--[:150])------))} % этилбензол
        \arrow(@benz.285--hep.west){<=>[][][]}[285,2,,,draw=none] % Линия к гептану
        \chemfig{(-[:35]-[:-35]-[:35]-[:-35]-[:35]-[:-35])} % гептан
        \arrow(@benz.325--hex.west){<=>[][][]}[325,2,,,draw=none] % Линия к гексану
        \chemfig{(-[:35]-[:-35]-[:35]-[:-35]-[:35])} % гексан
        \arrow(@benz.355--ihex.175){<=>[][][]}[355,2,,,draw=none] % Линия к изогексану
        \chemfig{(-[:-35](-[:270])-[:35]-[:-35]-[:35])} % изогексан
        \arrow(@benz.55--mcp.235){<=>[][][]}[55,2,,,draw=none] % Линия к МЦП
        \chemfig{([:234]*5((-)-----))} % МЦП
        \arrow(@benz.210--l.235){<=>[][][]}[210,2]
        \arrow(@benz.330--r.235){<=>[][][]}[330,2]
        \arrow(@tol.90--mch.225){s>[+(90:2) and +(210:1)]}[60,2] % curved arrows
        %\arrow(@[email protected]){s>[+(245:1.7) and +(90:2.4)]}[60,2] % curved arrows
        \arrow(@[email protected]){s>[+(249:1.7) and +(86:2.5)]}[60,2] % curved arrows
        \arrow(@ihex.90--mcp.330){s>[+(90:2.4) and +(330:0.8)]}[120,2.3]
        \arrow(@[email protected]){s>[+(306:1.5) and +(86:2.5)]}[120,2.3]
        \arrow(@hep.240--eb.340){s>[+(240:1.2) and +(340:2)]}[195,2.6]
        \schemestop
    \end{center}
    \normalsize
    \caption{Схематичное представление путей образования бензола в процессе каталитического риформинга}
    \label{fig:Benzene_precursors}
\end{figure}
\end{document}

Using the macro definition of the <=> and -> arrows, you could create similar bent alternatives. Like this:

\documentclass[a4paper]{article}

\usepackage[T2A]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage[english, russian]{babel}
\usepackage{amsmath}
\usepackage{graphicx}
\usepackage[colorinlistoftodos]{todonotes}
\usepackage{chemfig}

\begin{document}

\begin{figure}[h!]
\footnotesize
\begin{center}
\noindent
\makeatletter
\definearrow3{<§>}{%
    \CF@arrow@shift@nodes{#3}%
    \path[allow upside down](\CF@arrow@start@node)--(\CF@arrow@end@node)%
        node[pos=0,sloped,yshift= .866pt,xshift=-.5pt](\CF@arrow@start@node @u0){}%
        node[pos=0,sloped,yshift=-.866pt,xshift= .5pt](\CF@arrow@start@node @d0){}%
        node[pos=1,sloped,yshift= .866pt,xshift= .5pt](\CF@arrow@start@node @u1){}%
        node[pos=1,sloped,yshift=-.866pt,xshift=-.5pt](\CF@arrow@start@node @d1){};%
    \begingroup
        \pgfarrowharpoontrue
        \expandafter\draw\expandafter[\CF@arrow@current@style , bend left](\CF@arrow@start@node @u0) to (\CF@arrow@start@node @u1);%
        \expandafter\draw\expandafter[\CF@arrow@current@style , bend right](\CF@arrow@start@node @d1) to (\CF@arrow@start@node @d0);%
    \endgroup
    \CF@arrow@display@label{#1}{0.5}+\CF@arrow@start@node{#2}{0.5}-\CF@arrow@end@node%
}
\definearrow3{s>}{%
    \CF@arrow@shift@nodes{#3}%
    \expandafter\draw\expandafter[\CF@arrow@current@style, bend left](\CF@arrow@start@node) to (\CF@arrow@end@node);%
    \CF@arrow@display@label{#1}{0.5}+\CF@arrow@start@node{#2}{0.5}-\CF@arrow@end@node
}
\makeatother
\schemedebug{false}
\schemestart
\noindent
\setcrambond{2pt}{}{}
\chemfig{ % Benzene Center
   [:60]**6(------)} % Команда [:60] обозначает поворот молекулы на 60 град. Для ЦГ это незаметно. Однако baseline теперь внизу молекулы.
    \arrow(benz.north--ch.south){<=>[][][]}[90,2] % Линия к циклогексану
\chemfig{(*6(------))} % циклогексан
    \arrow(@benz.125--mch.305){<=>[][][]}[125,2,,,draw=none] % Линия к метилциклогексану
\chemfig{(*6(----(-)--))} % метилциклогексан
    \arrow(@benz.192--tol.east){<=>[][][]}[185,2,,,draw=none] % Линия к толуолу
\chemfig{(**6(-----(-)-))} % толуол
    \arrow(@benz.226--xy.10){<=>[][][]}[219,2,,,draw=none] % Линия к ксилолу
\chemfig{(**6((-)----(-)--))} % ксилол
    \arrow(@benz.255--eb.45){<=>[][][]}[248,2,,,draw=none] % Линия к этилбензолу
\chemfig{(**6((--[:150])------))} % этилбензол
    \arrow(@benz.285--hep.west){<=>[][][]}[285,2,,,draw=none] % Линия к гептану
\chemfig{(-[:35]-[:-35]-[:35]-[:-35]-[:35]-[:-35])} % гептан
    \arrow(@benz.325--hex.west){<=>[][][]}[325,2,,,draw=none] % Линия к гексану
\chemfig{(-[:35]-[:-35]-[:35]-[:-35]-[:35])} % гексан
    \arrow(@benz.355--ihex.175){<=>[][][]}[355,2,,,draw=none] % Линия к изогексану
\chemfig{(-[:-35](-[:270])-[:35]-[:-35]-[:35])} % изогексан
    \arrow(@benz.55--mcp.235){<=>[][][]}[55,2,,,draw=none] % Линия к МЦП
\chemfig{([:234]*5((-)-----))} % МЦП
    \arrow(@benz.210--l.235){<=>[][][]}[210,2]
    \arrow(@benz.330--r.235){<=>[][][]}[330,2]
    \arrow(@tol.90--mch.225){<§>[][][]}[60,2] % curved arrows
    \arrow(@[email protected]){<§>[][][]}[120,2.3]
    \arrow(@hep.240--eb.340){s>[][][]}[195,2.6]
\schemestop
\end{center}
\normalsize

\caption{Пути образования бензола}
\label{fig:Benzene_precursors2}
\end{figure}

\end{document}

The double arrows are still not super accurate, but at least the curve of both arrows is the same. And then, using bend is easier then using curves with coordinates, I guess. The above code yields:

---

Edit: Edited the yshift and xshift, so that they fit the 30 degree angle of bend.