2 in 1 out headphone amp

[jimilee]

Is there such an animal or can I cobble one together?

[RobA]

It would be pretty easy to knock one together. You could start with this one,
http://tangentsoft.net/audio/cmoy/misc/cmoy-tangent-sch.pdf
and add an input mixer based on inverting op-amp mixer stage.

You could convert the power supply section to one based on a 9V input and using a charge pump IC.