So a submarine trundling along as a passive listener is only visible to someone who, by looking for them, makes themselves visible.
Also, I believe navies have been asked to use their sonar less, because of the possible link between military sonar and mass whale and dolphin groundings.
They're running blind, but given the size of the ocean and the number of submarines, it must seem that the odds are on their side. Well, the odds are on their side. Probably. If submarines are in the habit of choosing the most efficient routes between places of mutual interest, that's going to shorten the odds considerably.
Either that, or I'd guess that there might have been some sort of military exercise going on to account for the submarines being so close together.
I doubt they use 'flight lanes' either - that would be too obvious a security risk. All you'd need would be a mole in Whitehall and the deterrent would be radioactive fish food.
Perhaps they were both looking for a crashed alien saucer in roughly the same place? I'm sure that must be it. :)
