overlay a tikz picture with another

Question

maybe a duplicate of how to overlay a tikz picture with another

but there is no solution

ive been trying to overlay a tikzpicture with another This code does not work

\setbox0=\vbox{\hbox{%
  \begin{tikzpicture}
    % ...
  \end{tizpicture}%
}}
%The width of the following picture is {\the\wd0}.
\box0
\hspace*{-\the\wd0}
\begin{tikzpicture}
% ...
\end{tizpicture}%

maybe it is of interest that the first picture is externalized?

If i used overlay the whole picture is randomly cropped to a 1000th or so.

Thanks guys for your support. Here is the mwe:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{external}
\usepackage{scalefnt} 
\tikzexternalize
\begin{document}
\tikzset{external/only named=true} % prevent unnamed graphics from externalizing - that is needed since externalizing strips annotations

{\scalefont{0.25}
    \tikzsetnextfilename{test}
    \begin{tikzpicture}
        \draw (-1,0) -- (1,0);
        \draw[blue] (0,-1) -- (0,1);
        \node at (5,5) {A};
    \end{tikzpicture}
} % scalefont
{\scalefont{0.25}
    \llap{
        \begin{tikzpicture}
            \draw[red] (-1,0) -- (1,0);
            \draw (0,-1) -- (0,1);
            \node at (-5,-5) {B};
        \end{tikzpicture}
    } % llap
} % scalefont
\end{document}

As you can see: The pictures are not overlayed I want to achieve that the 'cross' of both pictures is at the exact same coordinate. In the end, only the black/blue cross would be shown (since red/black would be overlayed) And both nodes would be shown as well (since none of them is overlayed)

Second edit:

\documentclass{standalone}
\usepackage{tikz}
\usetikzlibrary{external}
\usepackage{scalefnt} 
\begin{document}

{\scalefont{0.25}
    \begin{tikzpicture}[remember picture]
        \draw (-1,0) -- (1,0);
        \draw[blue] (0,-1) -- (0,1);
        \node at (5,5) {A};
    \end{tikzpicture}
} % scalefont
{\scalefont{0.25}
    \llap{
        \begin{tikzpicture}[remember picture,overlay]
            \draw[red] (-1,0) -- (1,0);
            \draw (0,-1) -- (0,1);
            \node at (-5,-5) {B};
        \end{tikzpicture}
    } % llap
} % scalefont
\end{document}

overlay and remember does not work either (at least as I intend)

Edit 3: thanks AlexG

\documentclass{standalone}
\usepackage[table,dvipsnames]{xcolor}
\usepackage{tikz}
\usetikzlibrary{external}

\tikzexternalize
\begin{document}
\tikzset{external/only named=true}
\makebox[0pt][l]{
    \tikzsetnextfilename{tt}
    \begin{tikzpicture}
        \useasboundingbox (-10,-10) -- (10,10);
        \node[draw=green] at (0,0) {};
    \end{tikzpicture}
} % makebox
\begin{tikzpicture}
    \useasboundingbox (-10,-10) -- (10,10);
    \node[draw=green] at (0,0) {};
\end{tikzpicture}

\end{document}

It yields

Answer 1

If you want to exactly overlay both pictures, they should have the same bounding box coordinates.

Command

\useasboundingbox ... ;

can be used for this. It doesn't put any visible marks on the page but forces the object (here: a diagonal line) do be used for calculating the final bounding box.

In general, LaTeX-type boxes, such as \makebox, should be preferred over low-level box commands of the TeX layer.

\documentclass{standalone}

\usepackage{tikz}
\usetikzlibrary{external}
\tikzexternalize % activate!
\tikzset{external/only named=true}

\begin{document}

\tikzsetnextfilename{tt}%
\makebox[0pt][l]{\begin{tikzpicture}[node font=\tiny]
    \useasboundingbox (-5.2,-5.2) -- (5.2,5.2);
    \draw (-1,0) -- (1,0);
    \draw[blue] (0,-1) -- (0,1);
    \node at (5,5) {A};
\end{tikzpicture}}%
\begin{tikzpicture}[node font=\tiny]
    \useasboundingbox (-5.2,-5.2) -- (5.2,5.2);
    \draw[red] (-1,0) -- (1,0);
    \draw (0,-1) -- (0,1);
    \node at (-5,-5) {B};
\end{tikzpicture}

\end{document}