your first adapter does not split your stereo. it distributes stereo from 1 stereo to 2 stereo outputs (not to 2x mono L+R, which is what you really want). then you are using 2 stereo cables which obviously transmit an identical signal to the amp and speakers. you never separate left and right channels actually. both sides of your speakers receive exactly the same signal.
2 possibilities:
1 - buy a new adapter stereo 3.5 to 2x mono 6.35. this is the safest option. i recommend this, as Csound did. you may also want to buy 2 mono 6.35 cables, although your 6.35 stereo cables may work as well (but not certain, depends on the new adapter).
2 - open your 2 stereo cables on the jack adapter ends. on one, clip the wire for left signal. on the other, clip right. DO NOT DO THIS IF YOU DO NOT FEEL SAFE ABOUT IT AND KNOW WHAT YOU ARE DOING! CLIPPING WRONG CABLES MAY DAMAGE YOUR AMP AND SPEAKERS! this is the fastest and cheapest option.
since i do not quite understand what cables you are using, better check correct wiring in the manual HS7/8:
https://usa.yamaha.com/files/download/other_assets/4/329474/HS8_en_om_a0.pdf