What's better, one conductor or two conductors for an RCA interconnect?

Since you are only concerned with single ended RCA interconnects with coaxial cables, I will share my experience with you.  

For many years I used a fine 75-ohm Belden #9259 oxygen free single center conductor coaxial cable with a bare copper braided shield over a polyethylene foam dielectric.  This was my principal preamplifier output to my mono block power amps input for many years.  I really had no complaints.  Due to a change in my room arrangement, I was forced to construct new & longer interconnects.  I was unable to acquire an adequate amount of the Belden #9259 cable without purchasing an entire roll.

I did however receive a timely tip from a respected analog guru.  He recommended Mogami W2549 coaxial cable which has 2 OF center conductors + bare copper braid.  You can experiment with it in 2 methods: 1) solder 1 center conductor to the RCA + cup, the other center conductor to ground & the bare copper braid as the shield or 2) solder both center conductors to the RCA + cup & the bare copper braid as ground.  Interestingly, the company that I purchased the cable from (Redco) actually terminates them per method 1 (above).  However, I chose the second method.

I made my main pre to power amp interconnects 25' long.  2 more pair 1 meter long and a 4th pair 2 meters.  After about 300-400 hours of "run-in" I was more than pleased.  Doing this project requires some good tools, materials and diligence or alternatively, pay someone or business to do it.  If you are a DIY person, you may wish to select an RCA connector with a crimp connector option for the braid so that you only have to solder the center conductor(s).  I found that soldering the braid to the ground of the RCA connector to be the most difficult part of the process, mostly because the RCA's that I chose didn't leave me much real estate to work in.

I was not able to compare these particular Belden vs Mogami coaxial cables in my system fairly, but I am very pleased and settled with this decision & execution. The Mogami cable is exceptional in every aspect with vivid detail and dynamics.  

I hope this has provided some limited comparative insight.

Best luck

I like Mogami too. I have their 3173 Cable as a interconnect between amplifier and preamp. This is a little thicker than the 2549 and also has a drain wire Plus 2 conductors. In this situation the drain wire is connected to both sides.

Still breaking in and thus far it’s a wonderful cable and very inexpensive.

I've heard inexpensive star quad from Straightwire that Excell. 

So how does the RCA connector accommodate 2 connectors(positive and negative) and a ground wire.

@jumia Simply by careful soldering.

So how does the RCA connector accommodate 2 connectors(positive and negative) and a ground wire.

^Twist them together.^

I put together some XLR to RCA yesterday.

On the Neutrik XLR:

• The two clear wires twisted together to pin 2.
• The two blue wires twisted together to pin 3.
• The shield all twisted up and soldered to pin 1. 

On the RCA:

• Clear wire pair twisted (XLR pin 2) to RCA center
• Blue wire pair twisted (XLR pin 3) to RCA shell.
• shield trimmed back, and not touching the shell at the RCA.

On the DAC I split the power and shoved a ferrite core on, with the +V in the one direction and the -V wound the other way.