There are a number of reasons why you shouldn’t share the same Auth code across multiple devices.
The main reason is that it’s impossible (or at least very, very difficult) to control the maximum number of read/writes to the server per second when two separate devices are doing independent read/writes. Get this bit wrong and you’ll be getting inconsistent results with no simple way of identifying the cause.
Secondly (although this is not an issue the way that you are doing it), the Blynk library/server combination will not fire the BLYNK_WRITE(Vx) callback when the update to pin Vx was from the same device, as it could cause an infinite loop.
Thirdly, the on/offline status information for devices in the app doesn’t work current try. If any one of the devices with the shared Auth code are online, the status in the app will show as online.
It’s not that difficult to use the bridge functionality provided by Blynk to allow communication between multiple devices, so why not use that functionality?