Il faut mettre les deux (`above left`) ensemble :
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
\node at (0,0) (a) {a};
\node (b) [above left=3mm left=2mm and 2mm 3mm of a] {b} ;
\end{tikzpicture}
\end{document}
