On the Medtronic FAQ's
However it's not clear if you can connect the sensor to the pump AND your Apple watch at the same time. I would guess it's an either / or thing as not all that many bluetooth devices are designed to be connected to two things at once. Plus I also imagine there would be significant battery drain on the transmitter IF it could be connected to two devices at the same time.
"How do I pair my transmitter with my iOS device?
You can easily pair your transmitter with your iOS device if your Bluetooth is turned on. In the Sensor Set up page of the Guardian Connect app, you can click ‘Pair Transmitter.’ The app will search for nearby transmitter and display the applicable device. You can select the device to pair, click on the device number, and pair within just a few moments."