Will line conditioner do anything for the other receptacle in a duplex?

Would it provide the same purported benefit as the Venom Defender, Furutech NCF Clear Line or Magnetic Wave Guide products?
No I agree with Jason, although I have not tested it. However there are a number of wall wart devices that you plug into a duplex receptacle which are supposed to help reduce noise everywhere, or in the adjacent plug. Based on reviews I have a few of these around the house. I haven’t done any detailed testing. At some point I found it just isn’t worth testing personally.
If that is the case then at some point you might want to try some things that actually work. Because I have them, and there is no "testing" you plug em in and hear the improvement immediately.
It should, as the these pieces are designed to take a "dirty" incoming signal, and send out a "clean" one.  The "clean" signal it sends out goes to your gear *AND* back to the outlet/duplex, hence a "clean" duplex (theoretically a whole circuit).  The effect will *not* be as strong on the duplex (or circuit), but you should get a little bump.Cheers!
The purpose of the module type power conditioner is to clean the AC line from the receptacle to the service panel. That includes both duplexes which are connected.
 Often, multiple modules are used in separate duplexes to reduce noise on other lines which will meet at the panel. This will have an cumulative effect. 

A line power conditioner is plugged into a receptacle, it's function is to reduce noise from the mains to all components which are plugged into this unit. It is only cleaning the AC for your audio system, it does not send a cleaned return signal to the panel.

There are so many different power conditioners, each with it's own unique design.

I have multiple in my panel, step down transformer, conditioner, phono stage, amp, speakers, crossovers, several drivers, and super tweeters. 
If a modular device (e.g., Venom Defender) and a whole house device (e.g., EP-2050) can "clean" the adjacent circuits, how is a power conditioner different?
@millercarbon could you recommend a brand. I would like to try one, starting with just one receptacle. Thanks!
I'm going to recommend what I just bought: Dave Elledge's P.I. Audio UberBuss.

Best conditioner around. And fair price as well.
Clearly I really need to stack my Bright Star Little Rock on top of my Puritan PSM 156.
The Puritan PSM156 or PSM136 will do this. It filters out over the air signals like WiFi, cellular, and Bluetooth. It also filters out noises from DC, compressors, and switch mode power supplies. It is incredible. If you are in the Chicagoland area this store lets you try before you buy. https://holmaudio.com/
They also have the Guardian power plug filter. 
My understanding is that the EP -2050 (https://store.acousticfrontiers.com/products/environmental-potentials-ep-2050) and perhaps the modular devices operate by serving as the preferred conduit for the noise in the signal. The EP-2050 functions in a similar way for surge protection. Any backwash, RFI, etc. that makes it to the distribution point (panel/outlet) Is essentially siphoned off preferentially, making it unavailable to other lines on the same circuit. This would be all circuits on the main panel for the EP2050 and the other outlet in the duplex for the modular device.  

I would think the line conditioner would serve as the preferred conduit in my case, leaving a cleaner signal to the other outlet. 

Theoretically correct?

The EP-250 looks like a good device, although I have no knowledge of its performance.

I’m not sure you fully understand the benefit of this device. There is no preferred conduit, this power conditioner/surge protector "cleans" the noise in all the circuits at the panel. This includes dirty power from outside and noise which has been generated within the house (appliances, furnace). So, noise in all lines is reduced, possibly a modular conditioner is not needed.

The line conditioner plugs into a receptacle and starts its "cleaning" at this point. All components will have less noise and SQ should be improved.

The other receptacle in the duplex is being cleaned by the EP-250 conditioner. The line conditioner serves only the one receptacle, and it is only cleaning the components plugged into it.

I fully understand the benefits of and the principles behind the EP-2050 and a modular unit (e.g., Shunyata Venom Defender).  My question is whether a power conditioner has the same impact on the other outlet of a duplex, i.e., that it also functions in the same fashion as a modular unit.
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I reached out to Audioquest who makes my power conditioner and this was their reply:

“Just having the Niagara 1200 connected to the wall outlet will help to improve any component plugged into the second outlet in the duplex.”



If true then this tells me something about the Niagara 1200. Because there are things out there that work this way. Hate to say, this also means not only the second outlet in the duplex but all other outlets in the house. Good luck figuring that one out. We report, you decide. 
A Richard Gray’s Power Company will have its full effect when plugged into the other receptacle.

Let me describe an unusual application of a Power Company: I was living in an old house that had several 120 volt window air conditioning units. The compressor was going bad on one unit, such that it would trip the circuit breaker every time it tried to cycle on. I didn’t feel like buying a new air conditioner at the time, as we were moving soon, so I plugged a Model 400 unit into the other receptacle. The compressor tripping was ELIMINATED, but returned soon as I unplugged the Power Company.   So it remained in place until we moved,

I’m not an expert on everything that’s available in the world of power conditioners, but I bet there aren’t many that could do THAT.

Ironic. A Dayton SA1000 powering a  Romeyn DEBRA is what’s plugged into the other outlet.