Parallel DC Conductors - Need Multiple Grounds?
A is smart. B is not. 1 you are wasting a conductor. 2 no ground loop exists in A. Both ground connections are basically equal and act as a single path, and your setup won't know the difference between two 14 awg wires in parallel and one 12 awg wire. 

B also results in twice the current in the green wire, which means higher power loss. 

While two power cables do not need two ground returns, the facts of this setup show that it's better to use two. A ground loop normally happens when you have two power sources,  not just one source on two wires.