Most shops will have something similar and fairly inexpensive. Usually passive "resistance mixers". However, none of them can drive a headphone output (not even the powered ones) - you'd need to plug them into a separate amp with phone output. IINM, most headphone amps like the S-amp don't usually mix, so you're stuck with small-format mixers.
If you get stuck, I've got a
Behringer RX1602 line mixer doing nothing. Possibly a bit overkill for you, but it would also let you mix in other instrument or line-level stuff too (in stereo if needed) and it'll cost you far less than any other workable options I know of. PM me if you're interested.
epictring wrote
Using 2 sets of earpieces isn't amazing, I can't explain why exactly but I feel I struggle to hear both equally well enough.
It's a psychoacoustics thing. When sounds enter different ears with no crosstalk you can actually only focus on one at a time. Mix 'em together and pan them to centre or either side and the brain can hear both easily.
