In both WWI and WWII no sub ever fired at a anothr deeply submerged (below periscope depth) sub. Subs strictly attacked targets on the surface (fully surfaced or at periscope depth).

It might be theoretically possible to fire from one deeply submerged sub at another (say both running at 200ft), but I doubt it. The torpedoes could be fired. Direction would have to be estimated by sonar. It would be such a blind shot it would stand virtually no chance of hitting.

But the problem is that the torpedo would have to be set to run at an extremely deep depth setting. Torpedoes of those days ran without active guidance - they just kept the depth and direction they were set to. The depth setting was controlled by a sealed diaphram in the torpedo, which contained sea level pressure air. This diaphram was compared to the outside water pressure via a simple mechanism, which was hooked to the torpedo's horizontal planes, which adjusted depth accordingly (it is similar in principal to the mechanism of an aircraft altimeter, if you know how that works). This was actually a very accurate mechansim, holding depth within a foot or two in test shots. Depth settings were dailed in prior to firing according to the target - deep for big ones, shallow for shallow draft ships like destroyers.

The deepest setting I have ever heard of is 40 or 50 ft: enough to run under a deep ship like a battleship and allow a magnetic exploder (which actually rarely worked) to detonate a torpedo underneath the vessel for maximum damage.

The point is, torpedoes were designed for shooting at surface tagets, and I doubt the depth setting mechanism on the torpedoes had settings of for anything beyond a few tens of feet.

In other words, no, it was probably not possible because you could not set the torpedoes to hold such a deep depth..

But the shot is no more silly than any other part of U-571.