This might not apply here, but if I need two lines to curve together I first use the ink pen to draw a wide single line (with whatever tapering and smoothing I need) in the shape of the curve I want. I create a stencil of that line, and using the "guide" mode of my stencil, I trace it with a smaller tool. Since I'm using "guide" rather than "ruler" or "stencil", I have the option of using a tapering line (ink pen) to make the trace. I use this as a locked layer to create a reference or tracing template for layers above it that create my painting.