Thanks to John Kormylo in this question ("Clip an A3 tikz overlay to two A4 pages"), I managed to create a background in two halves for use on two consecutive pages.
Now I need to create symmetrical one in which the LH side is a mirror of the RH side, eg
This is for a whole landscape page, so should I do this as two separate pictures, or is there a way to create it all in one go?
If you have a pdf file with background, you can mirror it with:
\documentclass[landscape]{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[overlay, remember picture]
\node[inner sep=0pt, outer sep=0pt, anchor=north west] at (current page.north west)
{\includegraphics[width=.5\paperwidth, height=\paperheight]{example-image}};
\node[inner sep=0pt, outer sep=0pt, anchor=north west, xscale=-1] at (current page.north east)
{\includegraphics[width=.5\paperwidth, height=\paperheight]{example-image}};
\end{tikzpicture}
\end{document}
And if you have a tikxpicture, you can mirror it with a shifted and scaled scope. In this case you can apply different options to the mirrored picture.
\documentclass[landscape]{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[overlay, remember picture]
\begin{scope}[shift=(current page.south west)]
\draw[line width=2mm, red] (0,0)--++(.5\paperwidth,\paperheight);
\end{scope}
\begin{scope}[shift=(current page.south east), xscale=-1]
\draw[line width=2mm, green] (0,0)--++(.5\paperwidth,\paperheight);
\end{scope}
\end{tikzpicture}
\end{document}
