The Equation Curve function (parametric 3D sketch) can be used also to generate double helix (helix on helix), e.g. to make a model of a twisted-pair cable.
To model such assembly, just use the function "Equation Curve" (3D Sketch > Draw > Equation Curve) and then Sweep (3D Model > Create > Sweep) of a circular profile along this curve.
Fot the equation curve, use e.g. the following parameters:
x(t) = cos(1 rad*t*PI)*100+cos(1 rad*t*PI)*5*cos(10 rad*t*PI)
y(t) = 5*sin(10 rad*t*PI)+80*t
z(t) = sin(1 rad*t*PI)*100+sin(1 rad*t*PI)*5*cos(10 rad*t*PI)
tmin = 0 ul
tmax = 6 ul
In the equations, the value of "10 rad" represents the number of wire twists per each loop of the helix, "80" is the pitch (height) of a half-loop of the helix, "100" is the radius of the helix, "5" is the wire radius.
Using this 3D sketch and sweeping of a circular profile (2D sketch) perpendicular to the curve you can make one wire. The second wire (with different color) is only rotated and offset. The rotation angle is 180/"twists per loop" (here: 18) and the offset in the Z axis is "half-loop pitch"/"twists per loop" (here: 8).
Similarly you can create also other types of 3D models, e.g. cooling pipes, etc.