On a crash event (IMU-Acceleration-Spike) both devices should send their data to the other device.
Are you referring to two devices that are aware of each other's presence before the crash or do you envisage a situation in which two machines that are complete strangers suddenly become aware of each other after the crash?
If the latter it seems to me that both need to send out enquiry messages at intervals and see if they get a response. After all, there may not be any other device present.
(By the way I am not familiar with the details of BLE but I presume it can switch from sending to listening).
PS. I am assuming when you say "simultaneous" that you really mean "within the first minute after the crash"
I'd prefer to be quicker than a minute and aim more for the <10, maybe <5 seconds territory.