No one wants to use two sound cards, but there should be an option under "choose where to play sound" that lists your main speakers, SPDIF, bluetooth speakers, et cetera that let's you chose multiple outs at once. Where is this function? It's vital. Apple has this for their products. It's a glaring deficiency in Windows.
100% this comment.
This use case comes up a lot for me when gaming. I use headphones and want to play with my son or friend, so mirroring audio to another headset would be helpful. This should be easy for the user to do, with an option to select multiple output devices.