Use Edit Curves > Curve Fillet to create a curve between two independent curves that do not intersect.
Controls whether the fillet curve warps in the direction of one curve or the other. A value of 1 warps the fillet curve toward one curve, -1 toward the other curve. A value of 0 puts the fillet curve evenly between the two. (You can use a value below 0 in the Attribute Editor, not in the options window.)