I have a very wide table (code below). I would like to put a small gap between the data under "First Group" and "Second Group" - two of the main multicolumns column headers. I tried putting in a double pipe (||) between these two columns where I specify the longtable. It splits the table but it behaves very weirdly in the multicolumn rows. How can I make the table split cleanly from top to bottom, hopefully without having to wade through the miles of code looking for &s? Thanks for your time!
Code for my table follows:
\documentclass[6pt]{article}
\usepackage[portrait, total={5.45in, 8.5in}, top=1.25in, bottom=1.25in, right=1.25in, left=1.5in, centering]{geometry}
\usepackage{longtable}
\usepackage{bm}
\usepackage[table]{xcolor}
\usepackage[none]{hyphenat}
\usepackage[T1]{fontenc}
\usepackage[default]{cantarell}
\usepackage{booktabs}
\definecolor{tableShade2}{HTML}{F1F5FA}
\definecolor{tableShade}{HTML}{FAFAFD}
\definecolor{tableShade3}{HTML}{dddddd}
\definecolor{tableShade4}{HTML}{C2A5CF}
\definecolor{tableShade5}{HTML}{E7D4E8}
\definecolor{tableShade6}{HTML}{A1D99B}
\definecolor{tableShade7}{HTML}{D9F0D3}
\definecolor{tableShade1}{HTML}{FFEDA0}
\pagestyle{empty}
\renewcommand{\familydefault}{\sfdefault}
\renewcommand{\arraystretch}{1.25}
\usepackage{arydshln}
\newcolumntype{x}[1]{>{\raggedright}p{#1}}
\arrayrulecolor{tableShade3}
\setlength{\tabcolsep}{4pt}
\begin{document}
\begin{center}
\scriptsize{\textbf{A very wide table with two groups}}\end{center}
\setlength\LTleft{0in}
\setlength\LTright{1.25in}
\setlength\LTpre{-0.3cm}
\setlength\LTpost{0in}
\rowcolors{1}{tableShade2}{white}\scriptsize
\newcommand{\CTPanel}[1]{%
\multicolumn{1}{>{\columncolor{white}}r|}{#1}}
\centering
\begin{longtable}{lp{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}||p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}p{0.3cm}}
\hiderowcolors
&\multicolumn{9}{c}{First Group}&\multicolumn{9}{c}{Second Group}\\\cmidrule(lr){2-10}\cmidrule(lr){11-19}
something&\multicolumn{2}{c}{AB} & \multicolumn{3}{c}{ABCD} & \multicolumn{4}{c||}{ABCD EFGH}&\multicolumn{2}{c}{AB} & \multicolumn{3}{c}{ABCD} & \multicolumn{4}{c}{ABCD EFGH}\\\cmidrule(lr){2-3}\cmidrule(lr){4-6}\cmidrule(lr){7-10}\cmidrule(lr){11-12}\cmidrule(lr){13-15}\cmidrule(lr){16-19}
Characteristics & + & - & I & II & III & $L^{A}$ & $L^{B}$ & $H^{+}$ & TN & + & - & I & II & III & $L^{A}$ & $L^{B}$ & $H^{+}$ & TN\\
\specialrule{0.02em}{0.1em}{0em}
\endhead
\specialrule{0.02em}{0em}{0em}
\endfoot
\hline
\multicolumn{10}{l}{\textbf{The first subheading}}\\\hline
\showrowcolors
some variable &+&+&-&+&+&-&-&-&-&-&+&+&-&-&+&+&-&-\\\hline
some other variable &+&+&-&+&+&-&-&-&-&-&+&+&-&-&+&+&-&-\\\hline
\end{longtable}
\end{document}
Resulting table (after edits suggested by D. Carlisle):