If you consider that every time a Kaiju is killed between the walls the paired partner on the Bastioner's side of the worldline split also dies, it would make sense to say that the same is true of Bastioner's killed between the walls. So that would suggest that, in the battle where Yuuko's sister died, with the ending of: "All targets eliminated at 7:42 PM. Two survivors confirmed out of 16 deployed." There were 14 people in the other worldline who spontaneously died during that battle and ONE who died who was not an active participant in combat. So, presuming that the other side has figured out what is going on between the walls, you would think they wouldn't keep killing themselves...
The term used was "many", implying "not everyone" - though apparently all the Kaiju "pilots" are such doubles for whatever reason. But then there's been no indication the Organisation can actually detect whether someone has a parallel double anyway so they're probably just guessing based on verfiable synchronised deaths...
And given their MO and persistence - as this conflict has apparently been going on for generations - the "other side" can be assumed to not give a flying fuck about their losses, anyway. Why they're so dogged, or trying to invade in the first place, is a big question mark and presumably will be important sooner or later.
Has there been any statement about what the cost of the second wall being breached is?
This is literally all we have to go by so far. No word on whether that's based on past experience or pure speculation, either - or if it's not just a vague claim made to convey how important it is to stop the Kaiju in the interval zone for that matter.