what you are trying to do may require you to be a bit more physical. Windows may not be able to split audio between two output devices, but you have the power to split it as many times as you like. it seems that your usb device did not work as you wanted when you bought it. you’ll find it much easier to get rid of that and just split the motherboard outputs with cables you can buy at any big box electronics store.
for example: I just upgraded to a digital receiver with the option of having s/pdif or rca inputs for my new speakers. my old speakers take a direct stereo cable connection from the motherboard to their inputs.
Reading: Combine two audio sources into one headphone
In this case, both speaker systems are 5.1 channel surround sound, and both have some sort of built-in surround sound simulation for stereo sources.
The new speakers are connected to the sound card with an s/pdif cable. old speakers are connected with stereo cables. this means that windows is seeing the optical s/pdif output and my regular 5.1 channel stereo outputs as separate devices. I want to watch a movie in 5.1 surround sound and I want it to come out of both sets of speakers, without using surround sound simulation on either.
See also: Windows 11 Not Recognizing Wired Headphones [5 Easy Fixes]
Sacrificing the digital output (which is converted to analog at some point before it reaches your ear anyway) and sticking with analog allows you to trick windows into thinking it’s just sending audio to one device. but how do I connect the analog receiver to the stereo jacks, which are already used by old speakers?
3 stereo splitters, plus 3 stereo-rca adapters, plus 3 rca cables. Even if you have an additional stereo headphone output, don’t use it in this type of setup. windows won’t account for preamp loss when splitting the outputs, meaning you’d be messing with volume levels just to save yourself the cost of 1 splitter.
so I’m splitting my 5.1 channel stereo into 2 separate speaker systems. one with 3.5mm input and one with rca. windows thinks it’s sending audio to a single speaker system. obviously this means I will have some (50%) preamp loss. your signal-to-noise ratio can suffer greatly with this type of setup, depending on the quality of your cables, speakers, and onboard hardware.
the other solution
takes the old speakers (the ones that connect directly to my motherboard with stereo cables and no adapters) and use a series of adapters to connect them directly to my stereo receiver. in this case, I could use s/pdif or multi ch rca in, depending on my specific output needs. computer games, for example, are not encoded with surround sound; your computer cannot send a 6 channel signal over s/pdif, only encoded stereo signals which are then decoded by the receiver. so for some audio sources i will need a true 6 channel output from the mobo to my ears. this particular solution, however, can be risky, especially if you’re trying to split the speaker cable output. if it isn’t, you will be limited by the number of outputs the receiver allows you to have at the same time.
the third solution
See also: How to Connect a Headphone Amplifier to an Audio Interface
connect the old speakers to my mobo and then connect your subwoofer’s rca outputs to the receiver’s multichannel input, instead of the satellites. The sats then connect to the receiver’s rca outputs, while the new speakers remain on their +/- cables. this is essentially using the receiver to do what windows can’t do. also risky because now I have a preamp on the inputs of my receiver. windows preamps the speaker volume to 100% (no dividers this time) and the volume from the old speakers will now preamp the receiver volume, which goes out to my new speakers. and I’m still limited by how many outputs my receiver supports at the same time. in my case this is stereo rca + single sub rca output == the surround data only comes from the new speakers, with the subwoofer and stereo output going to the old speakers. your mileage may vary on this. just make sure you keep your “preamp” as low as possible.
Conclusion: All operating systems have limitations; not all constraints have workarounds. if you’re willing to fiddle with some cables, you can do some crazy things with audio.
p.s. after reading some of his comments about what he’s trying to do, ceving, I’d say she just has to look at it the other way around. you say you want starcraft battle sound from headphones and “everything else” from usb speaker. well, depending on how many things are included in “everything” that is totally doable in the software. set the headset as the primary audio device. then change the media software output to usb speaker. You need to tell me what “everything else on speaker” means first before I can tell you what specifically you need to change. basically anything that isn’t starcraft should have an option to play through a usb speaker for what you’re thinking of to actually work.
Also note that windows does weird things with the audio device when it comes specifically to headphones. If possible, don’t tell windows that they are headphones, tell them that they are stereo speakers. tell windows that the usb speakers are a separate set of stereo speakers. that will put you at a starting point. Please note that to play stereo (2 channel audio) through 2 separate devices in windows, you just need to disable “mute speaker when plug in headphone”, plug one set to main jack, another set to headphone jack and ready. Good to go. I hope that’s not what you were trying to do because it would have been very simple. and i hope you’re talking about starcraftii because starcraft1 is actually set up in such a way that you get in-game music from frontleft/frontright and sfx out of the center channel when windows is in a surround setting which really is a whole other explanation .
See also: Soul SL150CB Pro Hi-Definition On-Ear Headphone review – The Gadgeteer