Well, they needed both drones to locate the source of any sounds detected in the water. With only one drone, their tests can determine what sounds were made (and maybe some general vicinity), but not as accurately from where.

The Chinese already surprised a US carrier fleet by deploying a submarine fleet that "ambushed" a carrier when one of their subs surfaced in kill range to the carrier, undetected.

In international waters? I'd call it piracy.

As is often the case in international politics, the justification is "well, what are you going to do about it, then?"

You are mistaken. Russia maintains survey vessels as well and regularly patrols just outside the US's exclusive economic zone.