The best way to connect your two centres is in parallel (+ to + and - to -) provided your receiver can handle the combined impedance. Two 8 Ohms centres result in a 4 Ohm combined load. Most receivers nowadays can handle this provided you don't have the receiver operating near its limits. You can also connect them in series but you will have to turn the centre volume up to compensate.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated