Merc:
The concert may have had 2 speakers in the front for amplification/reinforcement , but the sound from those 2 speakers bouncing off the side and rear walls provided the "rear surround channels" ambience. There's no way you could have heard the concert in 2CH, unless you had sound absorbing material 2/3's surrounding your head! Even then, it really would have been 3CH (LCR).