Symbol for Fourier pair as per Brigham, "The Fast Fourier Transform"

You could build something from a picture environment.

\setlength{\fsize}{\f@size pt}
        \roundcap\linethickness{#1 pt}
        \put( 0,.5){\line(1, 1.5){.25}}
        \put( 0,.5){\line(1,-1.5){.25}}
        \put(.1,.5){\line(1, 1.5){.2}}
        \put(1.5,.5){\line(-1, 1.5){.25}}
        \put(1.4,.5){\line(-1, 1.5){.2}}
\( \displaystyle a \Fpair b \)
\( \textstyle a \Fpair b \)
\( \scriptstyle a \Fpair b \)
\( \scriptscriptstyle a \Fpair b \)

The closest thing to that in Unicode is ⏣, the symbol for a benzene ring, which is \benzener in unicode-math, stix or stix2. You could define it as \mathbin{\benzener} or even stretch it out horizontally with a \scalebox.

Or you could draw it in TikZ.

From the suggestion of very good user @Davislor, I have created a symbol named \benz not very similar to your picture.

% create the node
\node[draw=black,minimum size=.7cm,regular polygon,regular polygon sides=6] (a) {};
\node[draw=black,minimum size=.55cm,regular polygon,regular polygon sides=6] (a) {};

$a\benz b$

enter image description here