Not all pings ar HF .. long range pings must to be in audible range, well in frequency range of the sphere sensor (and even the other sensors). In some Clancy book I remember raeding about 'chain of blips' on waterfall representing pinging active sonobuoys.
Check the pictures here (near the bottom of the page):
http://www.subguru.com/multimedia.htm
You either want to get close to get some info or you are just playing brave american soldier. Then you realize that the russian guy can play brave too.
I guess IRL really at least one of the subs, but most probably both knows the other sub is so close.
In DW I almost crashed once into Kilo .. it was at 0kts and gave exactly NONE sound. I had aged track and I just could not believe it is gone. I was snaking with HF sonar and when I saw it I had just time enough to avoid it.