... then there is a the Curve Stitcher plugin.

Select the 2 curves, Plugins -> Curve stitcher

Just be aware that it stitches from the start of the curve to the start of the other curve, so if you do the arcs in opposite directions you can get shapes like this

http://u1.ipernity.com/9/66/23/3226623.5467cce5.240.jpg