This style of fitting involves fixing 2 handles together with a shared screw/s. 
Recommended where a handle on both sides of the door is required e.g. an entry door which requires both an external handle and an internal handle. 

Round and most square handles require different back to back fittings.

Round handles: are center fixed and supplied with a double ended thread. The two handles simply wind together and the fixing hardware is completely concealed.

Installation instructions for center fixed back to back handles

Square handles: require 2 or more fittings located at the top, bottom and sometimes the center. These are supplied with screws and matching timber cover buttons to conceal the fixing hardware.

Installation instructions for multi-fitted back to back square handles