Amer, it's unnecessary to use the term "pseudo" because there's no "real" 7.1 format. Both DD EX and DTS-ES have a matrixed or discrete 6th channel, which is then reproduced by either one or two back surround speakers. It's generally agreed that two speakers do a somewhat better job with the 6th channel. As you see, the term "7.1" is something of a misnomer.
The two back speakers can be run off one amp by being connected to it either in parallel or in series. If 8ohm speakers were connected in parallel the resulting impedance would be 4ohms, which might be okay. In series, the impedance of the speakers would be doubled, but such a connection tends to result in some response variations, which might not be especially significant in a surround speaker. So, the only difference is that you'd be using one amp channel rather than two to drive the two rear surrounds.
