Deo, I've been doing DIY electronics for a while now. A Y-cable may work, but is risky, for reasons I probably don't have space to explain adequately, but basically it's possible to accidentally fry the output of one output by combining it with another output, without isolating each from the other. Ask me how I know :(
The next best solution would be to find or build a Y-cable that has resistors on the junctions to act as a static passive mixer, but
even better would be a simple stereo mixer. These are the best I could find on Amazon for as cheap as possible. You may be able to search around and find them cheaper on the eBay or other site. They have input jacks for stereo 3.5mm plugs, two adjustable inputs and one "straight through" main input so you can mix together 3 different devices. USB output also 8). Listed in order of price from ~$33.00 to ~$50.00:
Maker Hart "Just Mixer" Portable
https://www.amazon.com/Maker-hart-Just-Mixer-Portable/dp/B0824Q3WY8Maker Hart "Just Mixer" Desktop
https://www.amazon.com/JUST-MIXER-Audio-Mixer-Portable/dp/B01LPT8JY8/Maker Hart "Just Mixer 2" Desktop
https://www.amazon.com/JUST-MIXER-Compact-Powered-Desktop/dp/B071RZSGFR/This one is ~$26.00, but you'll need 1/4" adapters for all the inputs and outputs
https://www.amazon.com/ammoon-MX400-Ultra-compact-Channels-Adapter/dp/B01F6PTA04The perfect thing may be a standalone DJ cross-fader. It'll mix two sources in stereo with isolating electronics so you don't accidentally fry anything, and provide a convenient method to balance the volumes of each in one handy control, rather than trying to match the volumes at each device. The most inexpensive I could find is this:
https://www.amazon.com/Stokyo-Llc-MUS271202-KUTTER-Black/dp/B01JARADVO/ ~$50.00
It's passive, so no USB output, but also doesn't require batteries or power, and just the fader; no extra controls to complicate things.
Hope this helps!
EDIT: I just re-read your original post, and I missed a few things. You said you wanted no external power required; the first 2 "Maker Hart" devices can run on batteries, so that's an option. The Stokyo Kutter is passive, therefore needing no power, but is the most expensive at $50.00. Maybe not as helpful as I'd hoped :(
