tcolorbox, divided box

Question

I use tcolorbox. How I can achieve box like draft bellow?

--------------------
|Lorem   |ipsum    |
|        |         |
--------------------
|Lorem ipsum dolor |
|                  | 
--------------------

When I combine two boxes, I have space between them.

\documentclass[]{article}
\usepackage{lipsum}
\usepackage[most]{tcolorbox} %kolor wokół sekcji
\usepackage{geometry}
\usepackage{tikz}
    \usetikzlibrary{shadows}
\usepackage{tcolorbox}
    \tcbuselibrary{skins}

\newtcolorbox{FirstBox}[2]{
    sidebyside,sidebyside align=top,
    skin=enhanced#2,
    boxsep=1mm,left=1mm,right=1mm,top=1mm,
    colback=black!5!white,
    colframe=black!75!white,
    title={Box {\bf #1}}
}

\newtcolorbox{SecondBox}[2]{
boxsep=1mm,left=1mm,right=1mm,top=0mm,
colback=black!5!white,
colframe=black!75!white,
skin=enhanced#2,
}

%--------------------------
\begin{document}
\begin{FirstBox}{ABC}{first}
Lorem ipsum dolor
\tcblower
Lorem ipsum dolor
\end{FirstBox}

\begin{SecondBox}{ABC}{last}
\lipsum[4]
\end{SecondBox}

\end{document}

Answer 1

You can to play with before and after options trying to delete this white space but I think a better solution would be to build this construction inside a single tcolorbox.

The first part could be build with a tabular or two column environment.

But if you insist in a side by side tcolorbox you can insert a box inside another one.

\documentclass[]{article}
\usepackage{lipsum}
\usepackage[most]{tcolorbox} %kolor wokół sekcji
\usepackage{geometry}
\usepackage{tikz}
\usetikzlibrary{shadows}

\newtcolorbox{outerBox}[1]{
    breakable, enhanced,
    colback=black!5!white,
    colframe=black!75!white,
    title={Box {\bf #1}},
    top=0pt
}

\newtcolorbox{twocolBox}{
    sidebyside, sidebyside align=top,
    enhanced,
    arc=0pt,
    colback=black!5!white,
    colframe=black!75!white,
    boxsep=1mm,
    notitle,
    oversize,
    nobeforeafter,
    frame hidden,
}

%--------------------------
\begin{document}

\begin{outerBox}{ABC}
\begin{twocolBox}
Lorem ipsum dolor
\tcblower
Lorem ipsum dolor
\end{twocolBox}
\lipsum[4]
\end{outerBox}

\end{document}

Answer 2

This is one example with tabularx:

\documentclass{article}
\usepackage{array,tabularx,lipsum}
\usepackage[table]{xcolor}
\usepackage[most]{tcolorbox}
\newtcolorbox{MyBox}[1][]{%
    enhanced,
    boxsep=0pt,
    fonttitle=\bfseries\large,
    fontupper=\normalsize\sffamily,
    colback=yellow!10!white,
    colframe=red!50!black,
    colbacktitle=red!30!white,
    coltitle=black,
    before title={\noindent},
    #1
}%

\begin{document}
  \begin{MyBox}[tabularx*={\arrayrulewidth0.5mm}{X|X},title=Box ABC]%
    Lorem ipsum dolor & Lorem ipsum dolor \\\hline
    \multicolumn{2}{p{0.96\linewidth}}{\lipsum*[4]}
\end{MyBox}
\end{document}

While there are possibilities for improvement, this should serve as a starting point.

Answer 3

As long as there are rounded box corners there will be a slight separation at the border between two boxes at the corners.

The tcolorbox environment introduces top and bottom parskip values, which can be omitted by setting nobeforeafter, however, this indents the box a little bit. This can be prevented by using `\noindent.

\documentclass[]{article}
\usepackage{lipsum}
\usepackage{tcolorbox} %kolor wokół sekcji
\usepackage{geometry}
\usepackage{tikz}
    \usetikzlibrary{shadows}
\usepackage{tcolorbox}
    \tcbuselibrary{skins}






\newtcolorbox{FirstBox}[2]{%
    sidebyside,sidebyside align=top,
    boxsep=1mm,left=1mm,right=1mm,top=1mm,
    bottomrule=0pt,
    colback=black!5!white,
    colframe=black!75!white,
    nobeforeafter,%
    title={Box {\textbf{#1}}},
    auto outer arc,
    arc=0mm
    skin=enhanced#2
}%

\newtcolorbox{SecondBox}[2]{
boxsep=1mm,left=1mm,right=1mm,top=1mm,
colback=black!5!white,%
colframe=black!75!white,
nobeforeafter,
toprule=0pt,
notitle,
arc=0pt,
auto outer arc,
    skin=enhanced#2
}%


\begin{document}

\noindent
\begin{FirstBox}{ABC}{first}
Lorem ipsum dolor
\tcblower
Lorem ipsum dolor

\end{FirstBox}
\begin{SecondBox}{ABC}{last}
\lipsum[4]
\end{SecondBox}


\end{document}

Edit Improved(???) version using one \newcommand, doing all in one:

\documentclass[]{article}
\usepackage{lipsum}
\usepackage{tcolorbox} %kolor wokół sekcji
\usepackage{geometry}
\usepackage{tikz}
    \usetikzlibrary{shadows}
\usepackage{tcolorbox}
    \tcbuselibrary{skins}



\newcommand*{\SuperBox}[6][]{%
\begin{tcolorbox}[colback=black!5!white,colframe=black!75!white,  skin=enhanced,#1,title=#2]
\begin{tcolorbox}[sidebyside,sidebyside align=top,notitle,boxrule=-1pt,left=0pt,top=0pt,titlerule=0pt,leftlower=0pt,nobeforeafter,bottomtitle=-10pt,notitle]
#4%
\tcblower%
#5%
\end{tcolorbox}
\begin{tcolorbox}[boxsep=0mm,left=1mm,right=1mm,%
nobeforeafter,%
boxrule=-1pt,%
notitle]%
#6%
\end{tcolorbox}%
\end{tcolorbox}%
}%




\begin{document}
% [opt arg]{box title}{unused skin}{left box content}{right box content}{lower box content}
\SuperBox{Box ABC}{First}{Lorem ipsum dolor left}{Lorem ipsum dolor right}{\lipsum[4]}


\SuperBox[colframe=red,colbacktitle=red,coltitle={yellow}]{Box ABC}{First}{Lorem ipsum dolor left}{Lorem ipsum dolor \textcolor{blue}{\textbf{right}}}{\lipsum[8]}



\end{document}