documentclass[9pt,a5paper,twoside]{report}
usepackage{xcolor}
usepackage{blindtext}
%<<<<<<<<<<<<<<<<<<tcolorbox>>>>>>>>>>>>>>>>>>>
usepackage[skins]{tcolorbox}
tcbset{enhanced,%breakable
colback=white,colframe=green!30!black,
coltitle=white,
fonttitle=bfserieslarge,
drop fuzzy shadow southeast,
attach boxed title to top left={yshift=-0.5mm},%-0.3mm
boxed title style={size=small,colback=green!50!black,left=2mm,right=2mm,top=0.5mm,bottom=-0.5mm,sharp corners=south,arc=1mm,boxrule=1.4pt,},%boxsep=1mm,
boxsep=0mm, %padding
left=1.5mm,
right=1.5mm,
top=3mm,
bottom=3mm,
righthand width=4cm,
before={noindent},
arc=1mm,
outer arc=1mm,
sharp corners=northwest,
%title=#1%
segmentation style={black,solid,opacity=0.5,line width=1pt},
}
%<<<<<<<<<<<<<<<<<<Chap. Style>>>>>>>>>>>>>>>>>>>
usepackage[left=14mm,right=10mm,top=20mm,bottom=15mm,headheight=0mm,]{geometry} %left=1.5in
usepackage{fancyhdr}
pagestyle{fancy}
fancyhf{}
fancyhead[CO, CE]{{slshape{rightmark}}}
fancyhead[RO, LE]{thepage}
renewcommand{headrulewidth}{1pt}
renewcommand{footrulewidth}{0pt}
renewcommand{chaptermark}[1]{ markboth{#1}{} }
renewcommand{sectionmark}[1]{ markright{#1}{} }
usepackage[utf8]{inputenc}
begin{document}
%<<<<<<<<<<<<<<<<<<Beg. Doc>>>>>>>>>>>>>>>>>>>
begin{tcolorbox}[title=TcolorboX,sidebyside]
blindtext
tcblower
This is the lower part.
blindtext
end{tcolorbox}
end{document}