\begin{tikzpicture}[baseline=2ex]
\path (0,0) node (B') [shape=rectangle, draw]{\;$B'$\;};
\path (0,1) node (B) [shape=rectangle, draw]{\;$B$\;};

\draw [->] (B'.130) -- (B.230);
\draw [->] (B'.50) -- (B.310);
\end{tikzpicture}