How to convert single subwoofer wire to L/R RCA ?

Solution
Convert it to rca ?

No conversion necessary,just fit a single rca plug to each end.
Its pain with coax cable but absolutely doable,just buy decent wireable plugs.

You only need single subwoofer output on the reciever to either the left or right rca inputs on the sub (not both)

If youre currently running both plugs from the amp to the sub its completely unnecessary,thats a 7.2 amp, those outputs are for 2 subwoofers not a single connection to one - Id imagine only one output is even active unless youve purposefully enabled twin subs in the amp settings menu.

https://techsupport.cambridgeaudio.com/hc/en-us/articles/200551522-Why-are-there-twin-RCA-inputs-on-my-subwoofer-
Convert it to rca ?

No conversion necessary,just fit a single rca plug to each end.
Its pain with coax cable but absolutely doable,just buy decent wireable plugs.

You only need single subwoofer output on the reciever to either the left or right rca inputs on the sub (not both)

If youre currently running both plugs from the amp to the sub its completely unnecessary,thats a 7.2 amp, those outputs are for 2 subwoofers not a single connection to one - Id imagine only one output is even active unless youve purposefully enabled twin subs in the amp settings menu.

https://techsupport.cambridgeaudio.com/hc/en-us/articles/200551522-Why-are-there-twin-RCA-inputs-on-my-subwoofer-
 
Solution
To put it another way. You only need to connect one subwoofer output to one subwoofer input (if the sub has an LFE input then use that. If not it doesn't matter if you use left or right. Turn the frequency knob all the way up. Set the level control to 12 o'clock. These functions will be set in the receiver set up menu.
Both subwoofer outputs on the receiver are the same signal. Not left and right sub channels but two outputs for a single sub channel so you don't have to use a Y splitter to connect two subs. They label them .2 but they aren't stereo unless you can set them that way in the receiver menu, Not common to be able to do that.
 
@americanaudiophile - not important in this particular case but the Sony receivers do differentiate between the 2 sub outputs IF you have 2 seperate subs & use the Sony Mic & auto setup.
It will vary volume & crossover between the 2 slightly.