Two phones. One ball.
Carried between them by sound.
No Wi-Fi, Bluetooth or accounts. The two phones talk to each other by chirping, using the speaker and microphone.
Each phone is one half of the court. The top edge of your screen is a portal: the ball flies through it, the phone chirps it across, and it bursts out of the top of the other phone. You'll hear it go.
Knock the ball through a gem to load a trick. It goes off on their screen when the ball arrives:
Long rallies heat up the court. 6+ hits and the point is worth ×2, 12+ and it's worth ×3.
Every return charges your bomb meter. When it's full, tap while the ball is on their side to drop a bomb onto their screen, lined up with where you tapped. If it hits their paddle, the paddle is stunned.
Miss the ball and they score. Whoever misses serves next.