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

I would like to produce something like

S_1
===
S_2

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

$\underline{S_1}\atop\overline{S_2}$

(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:

$\underline{S_12345}\atop\overline{S_2}$

Yuk!

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

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