1. To place two posters on the same page:
Just do not leave a blank line between the two posters.
Indead, with TeX and LaTeX an empty line is interpreted as a new paragraph. So, the two posters are placed in two successive paragraphs.

\documentclass{article}
\usepackage{tcolorbox}
\tcbuselibrary{poster}
\usepackage{tikz}
\usepackage{lipsum}
\begin{document}
\begin{tcbposter}[
poster = {
columns=2,
rows=2,
spacing=3mm,
height=14cm,
width=6cm,
},
]
\posterbox[
colframe = red,
width=5cm, height= 10cm
]{ }{\includegraphics[width=3cm]{example-image-a}}
\end{tcbposter}% <- delete blank ligne
\begin{tcbposter}[
poster = {
columns=1,
rows=2,
spacing=3mm,
height=14cm,
width=6cm,
},
]
\posterbox[
colframe = blue,
width=5cm, height= 10cm
]{ }{\includegraphics[width=3cm]{example-image-b}}
\end{tcbposter}
\end{document}
2. My proposal to place the two posters:
Instead of placing two posters side by side, I propose to place only one of them, which is the size of the entire page where the top, bottom, left and right margins are removed.
coverage = {spread,left=0pt,right=0pt,top=0pt,bottom=0pt},
And we position within it, the two boxes that you wanted to place. The first with {xshift=1cm,yshift=-2cm }, the second with {xshift=7cm,yshift=-5cm }

\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\usepackage[poster]{tcolorbox}
\pagestyle{empty}
\begin{document}
\begin{tcbposter}[
coverage = {spread,left=0pt,right=0pt,top=0pt,bottom=0pt},
poster = {showframe,
columns=1,
rows=1,
rowspacing=0pt,
colspacing=0pt,
},
]
\posterbox[
colframe = red,
width=5cm, height= 10cm
]{xshift=1cm,yshift=-2cm }{\includegraphics[width=3cm]{example-image-a}}
\posterbox[
colframe = blue,
width=5cm, height= 10cm
]{xshift=7cm,yshift=-5cm }{\includegraphics[width=3cm]{example-image-b}}
\end{tcbposter}
\end{document}
3. With the textpos package
I didn't want to use the Tikz parameters that allow you to place a box precisely in a page because it requires you to compile twice, which is a problem especially if your document is complex.
I use another package (textpos) that allows to place text in an absolute position on a page, which LaTeX does not normally allow because it is designed to manage box spacing for us.
Here is the code of the final result:
\documentclass[12pt]{article}
\usepackage[a4paper]{geometry}
\usepackage[poster]{tcolorbox}
\usepackage[absolute%,showboxes
]{textpos}
\setlength{\TPHorizModule}{1cm}
\setlength{\TPVertModule}{1cm}
\pagestyle{empty}
\begin{document}
\begin{textblock}{6}(1,2)
\begin{tcbposter}[
coverage = {left=0pt,right=0pt,top=0pt,bottom=0pt},
poster = {
columns=1,
rows=2,
spacing=3mm,
height=14cm,
width=6cm,
},
]
\posterbox[
colframe = red,
width=5cm, height= 10cm
]{ }{\includegraphics[width=3cm]{example-image-a}}
\end{tcbposter}
\end{textblock}
\begin{textblock}{6}(7,5)
\begin{tcbposter}[
coverage = {left=0pt,right=0pt,top=0pt,bottom=0pt},
poster = {
columns=1,
rows=2,
spacing=3mm,
height=14cm,
width=6cm,
},
]
\posterbox[
colframe = blue,
width=5cm, height= 10cm
]{ }{\includegraphics[width=3cm]{example-image-b}}
\end{tcbposter}
\end{textblock}
\end{document}
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