Splitting a wide table or a longtable into two blocks

Question

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):

Answer 1

whenever you use a \multicolumn that ends in a column that has any | or @{...} material in its right hand edge, you need to re-insert it so if you have

|ll||ll|

then typically you will need

\multicolumn{2}{|c||}{heading for 1st 2 columns}&
\multicolumn{2}{c|}{heading for 2nd 2 columns}\\

This applies to all LaTeX tabular environments.

Answer 2

Based on David Carlisle's answer and comments, this hack gives a better impression of the longtable being split into two distinct blocks. It uses a wide vertical white rule to split the parts of the table. Hence it looks better if the all the table rows (or just alternate rows) are coloured with a different colour other than white.

\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}!{\color{white}\vrule width 5pt}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}}

lead to:

One more interesting fact I learnt through this question is that it is possible to make vertical rules play well with multicolumns as well as the tricky booktabs package by using the vrule option.

For example, this code

\begin{longtable}{lp{0.3cm}!{\color{tableShade3}\vrule}
               p{0.3cm}!{\color{tableShade3}\vrule}
               p{0.3cm}!{\color{tableShade3}\vrule}
               p{0.3cm}!{\color{tableShade3}\vrule}
               p{0.3cm}!{\color{tableShade3}\vrule}
               p{0.3cm}!{\color{tableShade3}\vrule}
               p{0.3cm}!{\color{tableShade3}\vrule}
               p{0.3cm}!{\color{tableShade3}\vrule}
               p{0.3cm}!{\color{white}\vrule width 4pt}
               p{0.3cm}!{\color{white}\vrule}
               p{0.3cm}!{\color{white}\vrule}
               p{0.3cm}!{\color{white}\vrule}
               p{0.3cm}!{\color{white}\vrule}
               p{0.3cm}!{\color{white}\vrule}
               p{0.3cm}!{\color{white}\vrule}
               p{0.3cm}!{\color{white}\vrule}
               p{0.3cm}!{\color{white}\vrule}
               p{0.3cm}!{\color{white}\vrule}}

leads to:

Not perfect, but functional and somewhat less ugly than the the | or || solution.