What is the purpose of the metal cable connecting the two brakes - not sure if it is 737NG specific. Have seen it called brake retention cable on a web search.

Not familiar with the NG, but others have a large tube running from one brake to another that keep the brake assemblies from turning. Think of it as the automotive equivalent to caliper bolts- something has to keep one part of the brake connected to the airframe.

The purpose is to retain the brake on the axle if a wheel assy. is lost (falls off). The brake just slides on the axle, it's not bolted on as the classic and jurassic 737's are. Instead the brake is retained by the MLG wheel and its braking torque is transmitted through a slot (cutout) in the brake to a torque pin on the landing gear. This allows for much easier and quicker brake changes. The cables are not NG specific, I'm aware of the MD-80's also using the same setup.