Most products with multiple outputs like that (see above) actually have multiple amps internally, to be intended to use with multiple headphones on multiple users simultaneously. If you want to use your own amp, then this is pointless.
I think what you're looking for is just a switchbox. Hook the output of your own amp to the switchbox input and your multiple headphones to the switchbox outputs. These are more common for RCA inputs and outputs, so you may need some adapters, but that should be okay. Or you could make one.