[Tex/LaTex] tcolorbox, divided box

tcolorbox

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.

enter image description here

\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}

Best Answer

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}

enter image description here

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