this is very easy to do.
open 'audio midi setup' in utilities.
click the plus in the lower left and add new aggregate device
check both display audio's
hit configure speakers in the lower right
check both checkboxes at top.
choose mutichannel & quadraphonic.
then you can click each one and test the sound.
hit apply and done
then right click on the aggregate device listed in the left you just created, click on use this device for sound output and player alerts and sound effects through this device
and you should be all set!!1 i had to configure levels on my displays, they were set differently. You can play with this (which i recommend) but clickin the triangle next to the aggregate device
click on the the first display audio and then on top click output and choose your levels.
do the same for the second display audio.
GOOD Luck