Cannot fathom why just one LED won't work with a particular library but will with another library

Troubleshooting
6

As I said, your "contradicting" scans will work due to the fact that they are mutually exclusively valid, but aren't you making things a bit too complicated for yourself?

You seem to have multiple code bases for (as I assume) rather similar use-cases.
But why not have the same code for both devices, you have the DEVICE_NAME to distinguish what's meant on the receiving end.

void SendStatus()
{
  char data[64];
  snprintf(data, sizeof(data), "A%d, B%d", state[A], state[B]);
  Particle.publish(DEVICE_NAME, data, PRIVATE);
}

On the receiving side

void handler(const char* event, const char* data) {
  int device = -1;

  if      (strcmp(event, "DeviceX") == 0) device = 0;
  else if (strcmp(event, "DeviceY") == 0) device = 1;
  
  if (device >= 0)
    sscanf(data, "A%d, B%d", &deviceStateA[device], &deviceStateB[device]);
}

Regarding the task to match the incoming device name against a list of possible options you can have a look at this post where another member asked me to elaborate a bit more about my proposal to use arrays and loops instead of individual variables