Not strictly a derivative, but in the same set as this thing: http://www.thingiverse.com/thing:7264

In building space frames that integrate solid rods, it's useful to have some varients, such as double sockets, and even triple sockets.

This thing provides the double socket varient. It's really good at helping you construct things like trestle bridges and other structures that need to fit multiple struts together, joining to a solid rod.

The default size on this part will slide onto a 5/16" rod, with a slight amount of friction. You can easily change the OpenScad file to fit whatever rod size you want.

It has the same sized holes as the single socket, and other socketed elements, so mix, match, and play around.

1) Look at your design for places where you are needing a solid rod, and a couple of flexible rods joined together.
2) Print a few of these double sockets
3) Slip them onto your rods in appropriate places
4) Firmly push sphere capped rods into the sockets
5) Rejoice!