Mixing 5.1 audio from two videos

Hi, I've read in brightsign audio related infos that if a player is playing multiple medias with sound toward the same output, sounds of the medias will be mixed before going out. I would like to know if it would be the same with two 5.1 audios coming from two videos played simultaneously on a hd225 as it allows to create two video zones. Setting up the audio outputs of the two zones as HDMI and passtrough would let me hear a mix of the audio coming from the two videos on my 5.1 system ?