Using (1) M-500t to power (2) center speakers?

I would like to use my M-500t that is currently running my center channel speaker from (1) channel only, to utilize both channels and run (2) center speakers. I would have to split the center out from the pre to the amp. Question, since this is how you connect for a mono set-up, will it be a problem using two speakers? The speakers would be connected normally as for stereo configuration. I'm not sure how the internal workings are in this amp. If I understand it right, the mono configuration comes from connecting the speaker cables to the non-inverting + terminal & the inverting - terminal. If I connect the speakers as in a stereo setup, I should be only sending the 251 WPC to the speakers. I need ideas on this. Thanks.

In th 60's, 70's, & 80's most preamps came with a mono switch. It would send both R/L inputs the same audio.

So your right with the Y connector is how you bridge the M-500T, But the M-500t wont care if the music input is mono, for all it knows it's still stereo.
It's when you wire your speakers up to your amp will the mono part comes into play. Just wire center Ch speakers as a stereo pair and all will be fine.

Thanks, I was hoping that would be the answer.