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

Single ended RCA cable, the ground is connected on both sides, there is no choice. If there is an additional shield, typically is it better to only connect it on one side.

@deludedaudiophile So, you are not EE.  Well, I am - designing low level electronics for 40 years.  Sorry I cannot explain it to you better.  We agree to disagree.

I have a PhD in materials science and close to 2 decades in semiconductors, semiconductor processing, and associate equipment, including forays into process measurement and control ... well exposed to signals in noisy environment. Doing much of the same in batteries now.  I understand the overall issues quite fine and could probably derive a lot from first principles. I understand your argument, I don't agree with all of it and I think you have poorly argued some points specifically as it may apply to real world applications including equipment ground connections and how they relate to loop resistance or impedance. Given the indeterminate ground connection and how that related to the loop resistance all due to the signal connector, I believe you are making erroneous conclusions about what would be common mode noise injection.

A speaker connection is balanced from the standpoint of the speaker, but if you induce noise on the speaker cables, it is not balanced at the amplifier input hence why the amplifier circuits for sensors and bridges, both often floating, are still fully differential circuits.

So it appears there are no clear Views on whether the ground wire should be connected to each RCA connector.  So you have 2 connectors connected to the RCA connects ans a ground wire connected to both RCA connects.  
 

So it is on this very point there is no clear rationale or consensus as to how the ground wire should be handled.

Personally I think this is really sad and should really frustrate those trying to choose the best interconnect.

Speaker by definition is balanced.  Noise currents induced in both wires cancel at the speakers, since there is no other return path.  If you question that, then perhaps you should read what Benchmark Media posted on their website in regarding to headphones being always balanced:
 

Headphone transducers respond to the voltage difference between the two wires that feed them. They have perfect rejection of common-mode interference because there is no path to ground or to any other conductor. In other words, there is no path for ground loops.

Headphone transducers are electrically isolated from everything other than the two wires that feed them. It doesn't matter if both conductors are driven differentially or if only one conductor is driven. The headphone transducer will reject common-mode noise.

The same happens when analog ground is floating on one side - no additional return path, hence inherently balanced connection with single ended output.  Analog ground is likely to be connected to chassis ground to reduce noise coupled from the chassis ground to circuitry, but it is often done with resistor of 100k or more.  It is because connecting analog ground to chassis on both ends directly or thru large capacitor creates perfect ground loop (earth ground - chassis - analog ground - interconnect - analog ground - chassis - earth ground).  That is why Benchmark amp doesn't even have single ended input.   This additional return path, possibly thru (two) 100k resistors creates some unbalance, but doesn't completely null usefulness of twisted pair in SE connection and that is why manufacturers use it.  For speaker wires it is no-brainer since it reduces noise, reduces inductance (important) and increases slightly capacitance (not-important).

As we continue about our qualifications, I assume that you've never designed any electronics?  As for me - I don't have PhD (only MSEE), meaning I'm still able to learn  :)    One more thing (Columbo) - it is "kHz", not KHZ.  I don't want to be unit police but I write it proper way automatically and it bothers me a little, when educated people don't pay any attention, sometimes using even erroneous terms like "watts rms"