Thank you for the suggestions.
I need something very small as it will be fit in a custom enclosure that houses two OB-6 modules. I don't need level tuning knobs or anything of that nature, since I can simply adjust the levels of the two OB-6 modules individually. (Well, I would still need a headphone level knob).
I've been reading that two Y-cables with a specific combination of resistors can do the job (but I've also read of potential small degradations in sound quality).
A very small powered mixer does seem like the best option, sound quality wise, but I'm not sure if there's one small enough for my needs.