[Bad5ector]

Depending on the AV receiver you might have the Bi-Amp option for the center channel, that would power both speakers at the same time and would have their own outputs. If you don't have a Bi-Amped center channel, you could I guess, double up on the speaker cable and combine the negative and positive ends of both cables and squeeze them all into the single center channel outputs on the receiver. The latter is pretty jank and probably wouldn't be good for both the receiver and the speakers long term. Probably best to stick with the single speaker as your center.

 

[Bad5ector]

Just checked your AVR's manual, looks like you can only Bi-AMP left and right front. Guess parallel is your only way to go if you wanted to use two, but if I were you I would stick with the one to be safe.