How to add a middle line between two paths?

I don’t know if this is a good solution for you, but I tried a variation of Myra’s idea of using Layer Styles. Instead, I used two Strokes. Both are set to Position: Inside. The first one is black, and the second one is a thinner white stroke that goes on top of the black stroke.

This could not be done with the graphic as uploaded because Layer Styles apply only to non-transparent areas. So first, I had to click Remove Background in the Contextual Task Bar so that the graphic would have only transparency around it. That gave the Stroke style something to outline properly.

If you applied this to images of different sizes or line weights, you would probably have to adjust the Stroke: Size option for each Stroke instance. But otherwise, you could save this setup in the Styles panel so that you can apply it to other layers in one click.

Es gibt mehrere Möglichkeiten.
Es ist nicht wichtig, ob es sich um Vektorformen oder Pixel handelt. Es auch möglich, entweder eine farbige oder eine transparente Mitte zu erzeugen.

Vektor (mit weißer Mitte)

Vektor (mit transparenter Mitte)

Pixel (mit transparenter Mitte)

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Konstruktion:

Effekt: Schein nach Innen mit Schwarz (aber benutze für eine weiße Mittenkontur einen Schwarz/Weiss Gradienten)

Mischoptionen (Regler für Schwarz etwas nach rechts schieben)

Viel Spass

I should add that the Outer Glow approach would be what you add to single stroke (the white in your example) and the Outer Glow would create the two black edges on either side.

You can apply the Outer Glow Layer Style, but you'll need to make some modifications to it. By default, the color applied to Outer Glow is white and the Blend Mode is set to Screen. Change the color to black and the Screen Mode to Multiply. You'll likely need to make the glow area larger, so increase the Size using the slider. It will still appear feathered, so increase the Opacity and decrease the Range. You might also try changing the Technique from Softer to Precise.