The Raspberry Pi+ LoRa(TM) Expansion Board has two RF modules. In my setup CE0 was 915MHz and CE1 was 433MHz so I modified the demo application so I could run both ports independently or simultaneously.

#if UPUTRONICS_RPIPLUS_CS0 && !UPUTRONICS_RPIPLUS_CS1
private const byte InterruptLine = 25;
private Rfm9XDevice rfm9XDevice = new Rfm9XDevice(ChipSelectPin.CS0, InterruptLine);
#endif
#if !UPUTRONICS_RPIPLUS_CS0 && UPUTRONICS_RPIPLUS_CS1
private const byte InterruptLine = 16;
private Rfm9XDevice rfm9XDevice = new Rfm9XDevice(ChipSelectPin.CS1, InterruptLine);
#endif
#if UPUTRONICS_RPIPLUS_CS0 && UPUTRONICS_RPIPLUS_CS1 // 433MHz and 915MHz in my setup
private const byte InterruptLineCS0 = 25;
private Rfm9XDevice rfm9XDeviceCS0 = new Rfm9XDevice(ChipSelectPin.CS0, InterruptLineCS0);
private const byte InterruptLineCS1 = 16;
private Rfm9XDevice rfm9XDeviceCS1 = new Rfm9XDevice(ChipSelectPin.CS1, InterruptLineCS1);
#endif

The in the run method

#if UPUTRONICS_RPIPLUS_CS0 && UPUTRONICS_RPIPLUS_CS1
public void Run(IBackgroundTaskInstance taskInstance)
{
rfm9XDeviceCS0.Initialise(915000000.0, paBoost: true, rxPayloadCrcOn: true);
rfm9XDeviceCS1.Initialise(433000000.0, paBoost: true, rxPayloadCrcOn: true);
#if DEBUG
rfm9XDeviceCS0.RegisterDump();
rfm9XDeviceCS1.RegisterDump();
#endif

rfm9XDeviceCS0.OnReceive += Rfm9XDevice_OnReceive;
rfm9XDeviceCS1.OnReceive += Rfm9XDevice_OnReceive;
#if ADDRESSED_MESSAGES_PAYLOAD
rfm9XDeviceCS0.Receive(UTF8Encoding.UTF8.GetBytes(Environment.MachineName));
rfm9XDeviceCS1.Receive(UTF8Encoding.UTF8.GetBytes(Environment.MachineName));
#else
rfm9XDeviceCS0.Receive();
rfm9XDeviceCS1.Receive();
#endif
rfm9XDeviceCS0.OnTransmit += Rfm9XDevice_OnTransmit;
rfm9XDeviceCS1.OnTransmit += Rfm9XDevice_OnTransmit;

Task.Delay(10000).Wait();

while (true)
{
string messageText = string.Format("Hello from {0} ! {1}", Environment.MachineName, MessageCount);
MessageCount -= 1;

byte[] messageBytes = UTF8Encoding.UTF8.GetBytes(messageText);
Debug.WriteLine("{0:HH:mm:ss}-TX {1} byte message {2}", DateTime.Now, messageBytes.Length, messageText);
#if ADDRESSED_MESSAGES_PAYLOAD
this.rfm9XDeviceCS0.Send(UTF8Encoding.UTF8.GetBytes("Netduino"), messageBytes);
this.rfm9XDeviceCS1.Send(UTF8Encoding.UTF8.GetBytes("Arduino1"), messageBytes);
#else
this.rfm9XDeviceCS0.Send(messageBytes);
this.rfm9XDeviceCS1.Send(messageBytes);
#endif
Task.Delay(10000).Wait();
}
}

#else

public void Run(IBackgroundTaskInstance taskInstance)
{
rfm9XDevice.Initialise(433000000, paBoost: true, rxPayloadCrcOn : true);
rfm9XDevice.Initialise(915000000, paBoost: true, rxPayloadCrcOn : true);
#if DEBUG
rfm9XDevice.RegisterDump();
#endif

rfm9XDevice.OnReceive += Rfm9XDevice_OnReceive;
#if ADDRESSED_MESSAGES_PAYLOAD
rfm9XDevice.Receive(UTF8Encoding.UTF8.GetBytes(Environment.MachineName));
#else
rfm9XDevice.Receive();
#endif
rfm9XDevice.OnTransmit += Rfm9XDevice_OnTransmit;

Task.Delay(10000).Wait();

while (true)
{
string messageText = string.Format("Hello from {0} ! {1}", Environment.MachineName, MessageCount);
MessageCount -= 1;

byte[] messageBytes = UTF8Encoding.UTF8.GetBytes(messageText);
Debug.WriteLine("{0:HH:mm:ss}-TX {1} byte message {2}", DateTime.Now, messageBytes.Length, messageText);
#if ADDRESSED_MESSAGES_PAYLOAD
this.rfm9XDevice.Send(UTF8Encoding.UTF8.GetBytes("AddressHere"), messageBytes);
#else
this.rfm9XDevice.Send(messageBytes);
#endif
Task.Delay(10000).Wait();
}
}
#endif

In the debugger output I could see addressed messages being sent to and arriving from a 433MHz Arduino device and a 915MHz Netduino device.

The thread 0x5f8 has exited with code 0 (0x0).
22:06:44-RX From Netduino PacketSnr 9.5 Packet RSSI -49dBm RSSI -110dBm = 20 byte message "Hello NetMF LoRa! 82"
22:06:47-RX From Arduino1 PacketSnr 9.8 Packet RSSI -56dBm RSSI -101dBm = 11 byte message "t 20.2,h 90"
22:06:48-TX 25 byte message Hello from LoRaIoT1 ! 163
22:06:49-TX Done
22:06:49-TX Done
The thread 0xe9c has exited with code 0 (0x0).
22:06:54-RX From Netduino PacketSnr 9.8 Packet RSSI -46dBm RSSI -111dBm = 20 byte message "Hello NetMF LoRa! 83"
22:06:57-RX From Arduino1 PacketSnr 9.8 Packet RSSI -61dBm RSSI -93dBm = 11 byte message "t 20.3,h 90"
22:06:58-TX 25 byte message Hello from LoRaIoT1 ! 162
22:06:59-TX Done
22:06:59-TX Done
The thread 0xf88 has exited with code 0 (0x0).
22:07:04-RX From Netduino PacketSnr 9.5 Packet RSSI -48dBm RSSI -110dBm = 20 byte message "Hello NetMF LoRa! 84"
22:07:07-RX From Arduino1 PacketSnr 9.8 Packet RSSI -61dBm RSSI -93dBm = 11 byte message "t 20.2,h 90"
22:07:09-TX 25 byte message Hello from LoRaIoT1 ! 161
22:07:09-TX Done
22:07:09-TX Done

This particular configuration has not been extensively tested yet and should be treated as early Beta (11 Sept 2018)