Why two inverters in parallel?
Those "logic ports" (I would call them "inverters", and some people call them "not gates") have limited current/power output. By connecting multiple units in parallel you simply multiply the current drive capability.  You can pull a heavier load with two horses (or mules or oxen) than with one.

And, as a separate matter, driving each side of the transducer with a full-voltage, but opposite phase signal also multiplies the actual power delivered to the load. That is called a bridge-tied load.  It is commonly found in audio amplifiers which operate on limited power. Most notably sound systems in vehicles which develop large amounts of power on the nominal supply of 12V.

Ref: https://en.wikipedia.org/wiki/Bridge-tied_load