You can create with a Helix the spiral, and while creating a Current State to Object from the Helix, use different Height Bias values to increase the width (middle) for the subtraction if the splines.
Create a Helix, make a copy.
Pull them in P.Y apart for the minimum distance.
Then set both Height Bias, so they create a more substantial middle part. Set one down and the other one up; in this way, the curve stays balanced.