Smoke, welcome. There is no such thing as an actual 7.1 format; the most is 6.1 on a few DD-EX/DTS-ES DVDs. SACDs and DVD-As are 5.1 or less. The reasons for possibly using two back surrounds rather than one are first, to avoid a psychoacoustic phenomenon which sometimes results in a sound which is supposed to come from behind you instead "reversing" and appearing to come from the front if there's just one speaker directly behind you. Dolby, THX, etc. all suggest using two back surround speakers even though there's only one back channel. Another reason is that DPLIIx and Logic 7(in some receivers)can be used to derive separate information for the 6th and 7th back speakers from both two and five channel source materials. So although there's no "real" seven channel material, the 7th speaker can still be useful.
