Back in the SH3 days, when ive met a convoy with medium or even better, light escorts i always tried to eliminate the escorts at one side of the convoy, which leaves time enough to shoot on the merchants...if the merchants werent armed i often tried to sink all escorts so i could fight the convoy surfaced with maximum dmg and maneuverability!
But shooting on multiple targets i never intentionally did iirc...for example yesterday ive met my very first merchant convoy in SH4, heavily guarded by small escorts but very chaotic formation, some ships moving, some not...i picked the nearest and biggest ship, fired 4 torps on it...one was a premature, and the ship tried to turn away from the other 3 torps, but 2 fishes hit it and the other one hit another small freighter perfectely!
A good way to practise that is by using free camera setting to control your actions better, and not to use manual aim so you can fully focus on things like multiple shots or other basic things like avoid escorts, keep the contact with the convoy etc