Components are cheap, and you can buy a decent soldering iron for the price of a bottle of mid-tier whiskey. The ability to solder and do some light troubleshooting also helps if you need to repair or modify your equipment (I added midi out to my Volca Keys).

Otherwise, if you just need a small passive mixer, you can find one on Amazon for the price of a cheap soldering iron.

The general rule is, splitting outputs is safe, may not always work, but not going to cook anything. Combining inputs into anything other than a mixer, likely unsafe.

Each signal is represented with an allowed range of current/voltage. Electronics expect signals in that range. Combining inputs, you sum the current/voltage and sometime get out of range values.

May be fine some of the time, but a strong signal from both sources and you'll start to hear a crackle as something in the signal path cuts out or fails to reproduce the pieces of sound out of the allowed range. Run it for a prolonged period with the crackle, it can cook speakers/headphones and anything else in the signal path (DSP in the headphones?). So, risk is gear dies an you need something new.

Just by a cheap line level mixer for $20. Something like this: https://www.amazon.com/Stereo-1-out-Passive-levels-control/dp/B08VJ44BSF/ref=sr_1_7?keywords=line+level+mixer&qid=1652505529&sprefix=line+level+mi%2Caps%2C108&sr=8-7