Brass Hickey Coupler 10mm 
The idea with this is to use 2 all threads to join any 10mm lampholder to this hickey.
You can then feed the cables through the center of the hickey to either lampholder.
Thread: 10mm x 1mm pitch
Hickey Gap between threads: 11mm
Please note there are not locks or stops on the thread so a dab of glue to prevent the thread from spinning once in place.