As we have all seen, most center speakers are MTM, meaning a horizontal configuration of a
Mid-Bass+Tweeter+Mid-Bass. That in itself is a less than ideal configuration.
If you use two bookshelf speakers for a center channel, you have to consider the orientation of the speakers. For example, if they are laying on their sides, will they be oriented as
Bass-tweeter-tweeter-Bass, or with the be
tweeter-Bass-Bass-tweeter or perhaps
Bass-tweeter-Bass-tweeter?
The ideal configuration for the speakers when placed horizontally would be
Bass-Tweeter-Tweeter-Bass. You want to place the tweeters as absolutely close together as you possible can to prevent phase cancelling and comb filtering. How much those problems will bother you personally in your circumstances is not predictable, maybe a lot and maybe not.
I'm in a vaguely similar situation. I have two large pair of speakers side-by-side. Not necessary for music but great for Home Cinema. I do get some Comb Filtering and some Phase Cancellation, but I feel that what I gain is more than what I lose, so I'm content.
As others have pointed out, there is the matter of Impedance. Two 8 ohm speakers in Parallel is 4 ohms. Your amp may or may not like that. Again, hard to predict.
If you wire the speaker in Series, the output will be the same as a single speaker, however, you will drive the speaker pair much more gently to get a given level of output. In fact, you will have roughly half the excursion for equal output, and twice the power handling capacity. But, two speakers so wired only put out sound equal in volume to one speaker.
I will try to explain this but it is a bit convoluted. Because you have two speakers you have twice the output. But because the speakers are wired in Series, each speaker gets half the signal.
So if
X is the output of a single speaker, and we now have two speakers, that gives us
2X. Now because they are in Series each speaker only get HALF (1/2) the Signal, so we now have
2X/2.
2x / 2 = X
X is the output of a single speaker;
X is the output of two speakers wired in Series. But at that output, you have half the excursion or cone movement so the speakers are straining much less to give the same output level. You have reduced the excursion by half and increased the power handling capacity. So even though the output is the same, you have gained something.
You amp, is far from high end, though reasonably far from the bottom as well -
Onkyo | TX-NR525
Onkyo is not known for its strong power ratings -
http://www.uk.onkyo.com/downloads/2/4/8/8/0/ONKYO_TX-NR525_datasheet_EN.pdf
* 130 W/Ch (6 Ω, 1 kHz, 1% THD, 1 Channel Driven, IEC)
* 160 W/Ch (6 Ω, 1 kHz, 1 Channel Driven, JEITA)
The fact that they are rating at 6 ohms and at only one frequency, and at pretty high distortion levels, and the fact they are using IEC and JEITA instead of Continuous power in another indication that they are fudging on the power ratings.
I'm going to guess the real power on this amp is about 80w/ch to 8 ohms full spectrum with 2-channels driven at reasonable distortion levels.
However, given that you will only drive one channel at 4 Ohms (2x center in parallel), and assuming your amp has adequate ventilation, this may not be a problem. I think it is certainly safe enough to try. If it does overheat, the amp will simply shutdown. I can't guarantee no problems, I can only say that I think you are not likely to have any problems.
Try both Series and Parallel wiring, and see if you like what you hear, but DO PLACE the tweeters as close together as possible.
Just one man's opinion.
Steve/bluewizard