only one sub per convoy route

[Treefrog]

I have one German sub near Canada and another near Ireland, both in hunt mode with 8+ supply, yet only the Canadian sub creates damage.
Did I miss a rule that only one sub may "hunt" on a given convoy route? Seems to me I recall the the surface raiders on "hunt" don't attack convoys if there is a sub on "hunt" on the same route?

Anybody have any experience with this or am I doing something wrong?

[Treefrog]

Answer: each may attack. A few turns after posting the questions the same wolf pack of three subs on the Canadian convoy all attacked.

[BillRunacre]

If one has done all the damage possible against an enemy convoy, then the other won't raid, so it's best to spread them over various convoy routes if possible.