It can be done with Audio Hijack Pro
You can hijack audio from a single application, or all audio, then use 4FX Effects in the Effects tab.
For example you can put Channel Tweaker and Auxiliary Device Output in both the top and second rows.
Set the top Auxiliary Device Output to the left display's audio and the other to the right display's.
Set the mode of he top Channel Tweaker to Duplicate Left, and the bottom to Duplicate Right.
This should put the left audio on both speakers of the left display and the right audio on both speakers of the right display.
For greater stereo separation, set the top to Kill Right and the bottom to Kill Left. This should put left audio on only the left speaker of the left display and right audio on only the right speaker of the right display.
Use Audio Hijack Pro's Mute button, so the audio comes only through Audio Hijack Pro.