\frac-like thing but with a pair of horizontal lines?

I would like to produce something like


with the "stretched equal sign" having rather close lines. At the
moment I'm using


(and $\displaystyle{\underline{S_1}\atop\overline{S_2}}$).

Is there a better way that will bring the two horizontal lines closer?
The closeness of the lines is a minor matter compared to the mess that
will result if the upper and lower texts differ in length:



The animated figures stand
Adorning every public street
And seem to breathe in stone, or
Move their marble feet.