Two-sided distance fields cost more to raymarch.
When you are getting pitch black spots on a solid mesh (especially in crevices like a staircase) it typically means your distance field resolution is not sufficient to approximate the mesh. The solution is just to increase the resolution scale until it goes away.
Enabling two sided distance fields isn’t necessary. They are really intended for porous surfaces with a lot of high density detail (such as foliage), not solid geometry.