Here is an idea for a new arrow type ->>
that allows to input both arrows as one. The arrow type will get 6 arguments:
\arrow{->>[above first]
[below first]
[above second]
[below second]
[ratio of arrow lengths (first:both)]
[label offset]}
The input
\schemestart
\scriptsize\chemfig{*6(-=-(-=_[:-30]-OMe)=-=)}
\arrow{%
->>[\footnotesize \textit{t}-BuLi, B(O\textit{i}-Pr$_3$)]%
[\footnotesize THF, -78$^\circ$C to rt]%
[\footnotesize KHF$_2$(aq)\footnotesize]%
[\footnotesize THF, rt, 31\%]%
[.57]%
}[,3.5,thick]
\scriptsize\chemfig{*6(-=-(-=_[:-30]-BF_3K)=-=)}
\schemestop
gives

This is done using chemfig
's \definearrow
. The whole code:
\documentclass{article}
\usepackage{chemfig}
\makeatletter
\definearrow{6}{->>}{%
\CF@arrow@shift@nodes{#6}%
\path (\CF@arrow@start@node) -- (\CF@arrow@end@node)
coordinate[pos=\ifx\@empty#5\@empty.5\else#5\fi] (mid@point) ;
\expandafter\draw\expandafter[\CF@arrow@current@style,-CF@full,shorten >=1pt]
(\CF@arrow@start@node) -- (mid@point);%
\expandafter\draw\expandafter[\CF@arrow@current@style,-CF@full,shorten <=1pt]
(mid@point) -- (\CF@arrow@end@node);%
\CF@arrow@display@label{#1}{0.5}{+}{\CF@arrow@start@node}{#2}{0.5}{-}{mid@point}%
\CF@arrow@display@label{#3}{0.5}{+}{mid@point}{#4}{0.5}{-}{\CF@arrow@end@node}%
}
\makeatother
\setatomsep{2em}
\setbondoffset{1pt}
\setdoublesep{3pt}
\setbondstyle{line width=1pt}
\renewcommand\printatom[1]{\fontsize{11pt}{11pt}\selectfont\ensuremath{\mathrm{#1}}}
\begin{document}
\schemestart
A \arrow{->>[a][b]} B
\schemestop
\schemestart
A \arrow{->>[a][][b]} B
\schemestop
\schemestart
A \arrow{->>[a][b][c][d]} B
\schemestop
\schemestart
\scriptsize\chemfig{*6(-=-(-=_[:-30]-OMe)=-=)}
\arrow{%
->>[\footnotesize \textit{t}-BuLi, B(O\textit{i}-Pr$_3$)]%
[\footnotesize THF, -78$^\circ$C to rt]%
[\footnotesize KHF$_2$(aq)\footnotesize]%
[\footnotesize THF, rt, 31\%]%
}[,4,thick]
\scriptsize\chemfig{*6(-=-(-=_[:-30]-BF_3K)=-=)}
\schemestop
\schemestart
\scriptsize\chemfig{*6(-=-(-=_[:-30]-OMe)=-=)}
\arrow{%
->>[\footnotesize \textit{t}-BuLi, B(O\textit{i}-Pr$_3$)]%
[\footnotesize THF, -78$^\circ$C to rt]%
[\footnotesize KHF$_2$(aq)\footnotesize]%
[\footnotesize THF, rt, 31\%]%
[.57]%
}[,3.5,thick]
\scriptsize\chemfig{*6(-=-(-=_[:-30]-BF_3K)=-=)}
\schemestop
\end{document}
chemfig
's schemes, i.e. the\schemestart ... \arrow ...\schemestop
? Can you please add a minimal working example (MWE)\mbox{}
still would leave a space between two\arrow
s in a scheme...