\documentclass[a4paper]{article}
\usepackage{german}
\usepackage[utf8]{inputenc}

\usepackage{pgfplots}
\usepackage{pgfplots.assert}

\usetikzlibrary{intersections}
\usetikzlibrary{pgfplots.decorations.softclip}
\usepgflibrary{fillbetween}

\begin{document}
\thispagestyle{empty}
\parindent=0pt
\parskip10pt

\begin{tikzpicture}
	\draw[name path global=A] (0,0) -- (1,1);
	\draw[name path global=B] (-1,2) -- (2,-2);
\end{tikzpicture}
$\to$
\begin{tikzpicture}
	% currently, this stuff has no bounding box...
	\useasboundingbox (0,0) (-1,2) (1,1) (2,-2);

	\fill[blue] 
		\pgfextra
		\expandafter\let\expandafter\A\csname tikz@intersect@path@name@A\endcsname
		\expandafter\let\expandafter\B\csname tikz@intersect@path@name@B\endcsname
		\pgfpathfillbetween{\A}{\B}
		\endpgfextra
	;%
\end{tikzpicture}



\end{document}

