voicemeeter – Mixing audio output between multiple devices on windows
description near the bottom of the page – https://superuser.com/questions/329147/how-to-use-two-audio-devices-simultaneously-with-windows-7
video tutorial – https://www.youtube.com/watch?v=lpvae_2WOSQ&feature=youtu.be
Asio4All?
Mataio Ford 