documentclass{scrartcl}
usepackage{chemfig}
begin{document}
schemestart
    chemfig{*6(------)}
    arrow{<->}
    chemfig{*6(------)}
    arrow{<->}[90]
    chemfig{*6(------)}
    arrow{<->}[180]
    chemfig{*6(------)}
schemestop
end{document}

documentclass{scrartcl}
usepackage{chemfig}

makeatletter
definearrow3{<->}{%
    CF@arrow@shift@nodes{#3}%
    path(CF@arrow@start@node)--(CF@arrow@end@node)%
            node[pos=0.5,sloped](CF@arrow@start@node1){}node[pos=0.5,sloped](CF@arrow@end@node1){};%
    expandafterdrawexpandafter[CF@arrow@current@style](CF@arrow@start@node1)--(CF@arrow@end@node);%
    expandafterdrawexpandafter[CF@arrow@current@style](CF@arrow@end@node1)--(CF@arrow@start@node);%
    CF@arrow@display@label{#1}{0.5}+CF@arrow@start@node{#2}{0.5}-CF@arrow@end@node
}
makeatother

begin{document}
schemestart
    chemfig{*6(------)}
    arrow{<->}[0]
    chemfig{*6(------)}
    arrow{<->}[-90]
    chemfig{*6(------)}
    arrow{<->}[180]
    chemfig{*6(------)}
schemestop
end{document}

definearrow3{<->}{%
    CF@arrow@shift@nodes{#3}%
    path(CF@arrow@start@node)--(CF@arrow@end@node)%
            node[pos=0.5,sloped,xshift=-1pt](CF@arrow@start@node1){}node[pos=0.5,sloped,xshift=1pt](CF@arrow@end@node1){};%
    expandafterdrawexpandafter[CF@arrow@current@style](CF@arrow@start@node1)--(CF@arrow@end@node);%
    expandafterdrawexpandafter[CF@arrow@current@style](CF@arrow@end@node1)--(CF@arrow@start@node);%
    CF@arrow@display@label{#1}{0.5}+CF@arrow@start@node{#2}{0.5}-CF@arrow@end@node
}