Android BLE App can ONLY subscribe characteristic notification for 1 BLE device

Question

I'm new to Android. I'm trying to subscribe characteristic notifications from 2 BLE devices. Now, my App can only receive the BLE data from the 1st device. I understand that BLE is serial communication, so I must wait until the onDescriptorWrite() callback is called before I enable the notification for the 2nd device. We can ONLY have 1 GATT operation each time. My questions are:

1. How to modify the onDescriptorWrite()? Thread.sleep() delay method doesn't help.

2. Some people mentioned to use queue to add/remove something?

    public void setCharacteristicNotification(BluetoothGattCharacteristic characteristic, boolean enabled,int device) {
    if (mBluetoothAdapter == null || mBluetoothGatt == null || mBluetoothGatt1 == null) {
        Log.e(TAG, "BluetoothAdapter not initialized");
        return;
    }
    if(device == 0) {
        mBluetoothGatt.setCharacteristicNotification(characteristic, enabled);
        BluetoothGattDescriptor descriptor = characteristic.getDescriptor(UUID.fromString(SampleGattAttributes.CLIENT_CHARACTERISTIC_CONFIG));
        descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
        mBluetoothGatt.writeDescriptor(descriptor);
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        Log.e(TAG,"Device #0 is done for notification!");
    }
    else if(device == 1){
        mBluetoothGatt1.setCharacteristicNotification(characteristic,enabled);
        BluetoothGattDescriptor descriptor = characteristic.getDescriptor(UUID.fromString(SampleGattAttributes.CLIENT_CHARACTERISTIC_CONFIG));
        descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
        mBluetoothGatt.writeDescriptor(descriptor);
        try {
            Thread.sleep(100);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        Log.e(TAG,"Device #1 is done for notification!");
    }
   

   }

    private final BluetoothGattCallback mGattCallback = new BluetoothGattCallback() {
    @Override
    public void onConnectionStateChange(BluetoothGatt gatt, int status, int newState) {
        String intentAction;
        if (newState == BluetoothProfile.STATE_CONNECTED) {
            intentAction = ACTION_GATT_CONNECTED;
            mConnectionState = STATE_CONNECTED;
            broadcastUpdate(intentAction);
            Log.e(TAG, "Connected to GATT server #0.");
            // Attempts to discover services after successful connection.
            Log.e(TAG, "Attempting to start service discovery #0:" + mBluetoothGatt.discoverServices());
   
        } else if (newState == BluetoothProfile.STATE_DISCONNECTED) {
            intentAction = ACTION_GATT_DISCONNECTED;
            mConnectionState = STATE_DISCONNECTED;
            Log.e(TAG, "Disconnected from GATT server #0.");
            broadcastUpdate(intentAction);
        }
    }
   
    @Override
    public void onServicesDiscovered(BluetoothGatt gatt, int status) {
        if (status == BluetoothGatt.GATT_SUCCESS) {
            broadcastUpdate(ACTION_GATT_SERVICES_DISCOVERED);
        } else {
            Log.e(TAG, "onServicesDiscovered received: " + status);
        }
    }
   
    @Override
    public void onCharacteristicRead(BluetoothGatt gatt,
                                     BluetoothGattCharacteristic characteristic,
                                     int status) {
        if (status == BluetoothGatt.GATT_SUCCESS) {
            broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic,0);
        }
        try {
            Thread.sleep(700);
        }catch(InterruptedException ex) {
            Thread.currentThread().interrupt();
        }
        Log.e(TAG,"onCHAR READ");
    }
   
    @Override
    public void onCharacteristicChanged(BluetoothGatt gatt,
                                        BluetoothGattCharacteristic characteristic) {
        broadcastUpdate(ACTION_DATA_AVAILABLE, characteristic,0);
        try {
            Thread.sleep(700);
        }catch(InterruptedException ex) {
            Thread.currentThread().interrupt();
        }
        Log.e(TAG,"onCharacteristicChanged #0 = ACTION_DATA_AVAILABLE, Done! ");
    }
   
    @Override
    public void onDescriptorWrite(BluetoothGatt gatt, BluetoothGattDescriptor descriptor, int status) {
        super.onDescriptorWrite(gatt, descriptor, status);
   
        // Do something here ????????????
   
        // Thread.sleep() delay method doesn't help
        try {
            Thread.sleep(700);
        }catch(InterruptedException ex) {
            Thread.currentThread().interrupt();
        }
        Log.e(TAG,"onDescriptorWrite #0, Done! ");
    }
   

   };

Answer 1

 mBluetoothGatt1.setCharacteristicNotification(characteristic,enabled);
 BluetoothGattDescriptor descriptor = characteristic.getDescriptor(UUID.fromString(SampleGattAttributes.CLIENT_CHARACTERISTIC_CONFIG));
 descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
 mBluetoothGatt.writeDescriptor(descriptor);

I think that should be:

 mBluetoothGatt1.setCharacteristicNotification(characteristic,enabled);
 BluetoothGattDescriptor descriptor = characteristic.getDescriptor(UUID.fromString(SampleGattAttributes.CLIENT_CHARACTERISTIC_CONFIG));
 descriptor.setValue(BluetoothGattDescriptor.ENABLE_NOTIFICATION_VALUE);
 mBluetoothGatt1.writeDescriptor(descriptor);