I think what I’m asking is… “What can I use to take the headphones outs from a Cycles and Samples so that I can listen to both from my headphones (1/4” or 1/8") when away from home?" I don’t really want another mixer so was hoping a specific cable would do it.
Have you checked the Roland go:mixer? its tiny and could work nicely for summing both boxes. @darenager sells a mini summing mixer too, but IIRC it only works for line level signals… last time I tried it with headphones no sound came out to the headphones.
With such an Y-adapter you effectively shorting two outputs together. At least you need some resistors in the signal lines to prevent any harm to the involved devices.
Check out Yamaha session cake as well! SC-02 (the two channel session cake model) has a TRRS aux minijack port and two 1/4" inch inputs + headphone amp. So you could plug one model: into the quarter inch inputs and another into the aux input.
I got one of these for portable/satellite setup. Works surprisingly well and has very clean sound, but does require a power source. Cheapest option I know of. One nice thing is that each of the 4 inputs can be either stereo or mono.
Thanks for the mention but mine are not suitable for headphone use as @tsutek said, headphones are typically 16-600 ohms, my mixers are line level (10k ohms)