I am confused. One or two rear speakers.

I think that this is the new 6.1 format.

This genre of Dolby Surround Sound, Dolby EX adds a rear center channel to the existing left and right rear channels. (I think it is also referred to as Dolby 6.1). I received the following explanation of the format direct from Dolby Labs:

As to your compatibility question, the Dolby Digital Surround EX format, which adds a third surround channel, is fully compatible with 5.1. The extra surround information is not on a separate, discrete channel, but rather is matrix encoded onto the left and right surround channels of an otherwise standard 5.1 soundtrack. You can at your option decode the extra channel using Surround EX equipment, or not. If you choose not to, no information is lost because it's all there on the standard two surrounds. Currently, Surround EX decoding is available exclusively on THX-certified home theater components.
Best regards,
Info @ dolby

This is the new ES system. It can be beneficial in filling in the rear area. There are several augmentations to this such as Lexicon's 7.1, which has 2 surrounds and 2 rear channels. However, right now there is very little software that I am familiar with that is native 6.1 or 7.1 channels. The rear channel(s) is derived from the other channels. The September/October issue of the Perfect Vision did a nice piece on disecting all of these new formats--it is confusing. For me, I'm sticking with 5.1 dolby digital and DTS until another format becomes a standard and not just a feature to differentiate a product in the market.