Question 3.5mm Y splitter Only outputs to 1 speaker in Pair (not both)

[Gamefreaknet]

In short my case splits the audio into 2 separate ports for mic and headphones. Whilst I do have a spare of those same ports on the back of my mobo I noticed my Y splitter (male to 2 female) only split the Audio to 1 of my speakers and the other speaker (same speaker model and connected through a wire to the other speaker - link here for Speaker).
Is this just a poor splitter or not possible not actually split like this?
I intend to try take advantage of the surround sound from my ASUS ROG Throne Stand (through my speakers)
Whilst my speakers do have bluetooth I have found there is some sort of interference (only with my PC not my other devices for some reason) which I suspect is the broadband in my Flats as when I looked up the brand (Hyperoptic) others had also noted bluetooth interference issues with them...

 

[Gamefreaknet]

Note have figured out the surround sound so it outputs through my headset/speakers but since my headset relies on USB-C to detect its mic a "whine" emits when connected both to the ROG Throne and my USB-C port (I tried an adapter to USB-C to one of the USB ports on the ROG Throne with no difference)