ZKFinger SDK connect to multiple ZK4500 in C#

Has anyone connected two or more ZK4500 fingerprint sensor to one PC? I used the ZKFinger SDK.

But when I try to connect from my applilcation, only one sensor is initialized.

Here's my code :

if(axZKFPEngX1.InitEngine() == 0) {
   MessageBox.Show(axZKFPEngX1.SensorCount.ToString());
   for(int i=0; i<=1; i++) {
       if(axZKFPEngX1.SensorIndex == i)
       {
          MessageBox.Show("Sensor : " + i + " - Serial Number : " + axZKFPEngX1.SensorSN);
       }
   }
}