The fact that they're both nuclear subs is frightening in itself. It seems funny, but imagine the environmental holocaust if there was a hull-breach on one or both vessels (in their respective reactor compartments no less).
I actually shudder to think of the consequences.
That they actually didn't detect each other is either a testament to super-stealthy acoustic signatures,
or technically subpar sonar capabilities on both vessels.
(...yes, I'm a fan of this sort of thing
)