Once twisted, the wires are not reflections of one another in one plane; they both wind in the same direction. If Z is along the helix, wire A is a rotation through 180 of wire B around Z, or A is B scaled by -1 in X and Y, (a reflection in 2 planes simultaneously, which the Mirror modifier doesn't do).
TLDR; You can mirror the wires before twisting:
Foreground, front to back, with the following modifiers:
- A straight Bezier curve, subdivided into a reasonable number of 'Automatic' control points, and offset in X from its object origin, in Edit Mode.
- Mirrored in X
- Simple Deform > Twisted through 360, around Z
- Arrayed in Z
Background: Curved around another spiral curve.
If you need two colors in the flex, you can offset the V of the UV by -1 in the Mirror modifier's 'Data' section, and use nodes something like this..
..testing for V < 0.