Apparently so, as Amazon offers such a cable. I couldn't tell you if it works.

https://www.amazon.com/CD-ROM-Audio-Cable-Spl … L/dp/B00GZHH0LE

You lose half the volume I guess?

The most elegant approach would be making a simple, passive mixer.
That involves a few capacitors, essentially.
These Y cables are nonsense from a technical point of view, I think.
Never connect two transmitters directly with each others.

The other way round (like in this thread) is acceptable, though. Two receivers (inputs) connecting to one transmitter (output).
That being said, maybe todays sound devices are more idiots proof, also. 😉
Seriously, who knows which safety measures they contain in their ICs?

PS: There's something to keep in mind:
The Mic-In port on soundcards provides phantom power for condenser microphones.
Please don't use that input without checking pinout first, never use a mono plug (shorts).
Ideally, use galvanic insulation here. A capacitor or transformer can be used to keep DC power away.
Even if DC is on a separate pin, both are preferable.

I use that cable in my setup, I haven't had any issues with it.

There's also the option of using a normal (analog) CD audio cable for one sound card, and a SPDIF (digital) CD audio cable for the second sound card, if applicable.

That's what I'm doing on this system. The SBLive is connected to the DVD drive using a SPDIF cable, while the ALS100 uses the normal, analog cable. Both work fine.

I see, thanks for your input guys! I think like a mixer is also an idea, also think about a small pcb with a switch.