As far as I know this is not possible through one sound card audio
settings although there may be mixer software that I am not aware of.
When I did some work for the RNIB years ago to solve this they provided a
hardware kit, a splitter box which forced Jaws through one channel and
allowed you to hear another sound input such as a telephone line through
the other channel of a headset/microphone setup. This was quite efficient
and I believe that this kit can still be purchased.
Jaws can be used with multiple sound cards, perhaps a USB as well as an on
board sound card. In theory you could have 2 sound cards and using a
splitter box force Jaws output into one channel and another card through the
secondary channel with such a kit. Of course As a much cheaper Heath
Robinson solution is to simply use 2 sets of tin ear earphones coming from 2
installed sound cards and simply put one earphone from each card in each ear
to hear the output from both cards at once.

David Griffith.