The thing is, Sony sub is passive, so it gets it power from the main unit. Adding a second passive sub woofer might stress out the amplification unit. and using a powered sub by using a Y splitter by using the HDMI out with an HDMI to RCA adapter. I think i got that right. You will have to match the crossover on the powered sub as the HDMI out will send all frequencies to the sub. Because the Amp unit has no preouts or LFE channel out.
also you might need a HDMI splitter to split the ARC out to 1.TV and the other to HDMI to RCA out.LOL! Recheck with someone when you do this, I am not sure how ARC behaves with splitters.