\newcount\letcount
\setcount\letcount=0 

\def\Let{\ifnum\count\letcount>0 \else\letit\fi\advcount\letcount by 1 }
\def\letit{\let\\=\cr}

\def\align{\Let\vbox\lb\halign\lb$##$&$##$\cr}   
\def\endalign{\cr\rb\rb\advcount\letcount by -1 }

$$ \align a&=\align x&y\\z&w\endalign\\c&d\endalign$$
\endinput
\def\Let{\relax\iffalse{\fi\let\\=\cr\iffalse}\fi}
$$ \align a&\align x&y\\z&w\endalign\\c&d\endalign$$
$$ \align a&=\align x&y\\z&w\endalign\\c&d\endalign$$