\begin{tikzpicture}[baseline=-1ex]
\node[fill=white, draw, rectangle] (left) at (0,0) {$\JW{2}$};
\node[fill=white, draw, rectangle] (right) at (2,0) {$\JW{2}$};

\begin{pgfonlayer}{background} 
\draw[fill=black!20]
	($(left.250)+(0,-0.5)$) -- (left.110) to [bend left=90] (right.70) -- ($(right.290)+(0,-0.5)$)
	($(right.250)+(0,-0.5)$) -- (right.110) to [bend right=90] (left.70) -- ($(left.290)+(0,-0.5)$);
\end{pgfonlayer}

\end{tikzpicture}