Tried blend but it simply puts a curve between the blunt ends, if these were arcing just a bit more and I wanted to keep the arcing flavour as opposed to sticking in a straight line, what command would create an arc in keeping with the existing arcs starting and ending tangent, i.e. feathering into the arcs just nice at the point where a tangent line would start and end , or thereabouts ?
Since your curves “dip” down toward each other like the top of a heart shape, there is no solution that will give you an arch shape between them that is tangent at the ends.
I suggest that you turn on the control points and the CurvatureGraph for both curves and start drawing a new curve, snapping to a few key control points of the old curves. Start the curves way back before they start to dip, skip the control points near the ends and then pick up the control points on the other side. Skip snapping to control points where the curvature graph dips and the control points near the ends of the existing curves.
Split the old curves at the ends of the new curve and join.