Ash,
You can run 2 x low power dongles on a Pi2B, but depending on your setup, (e.g. PAW Out to Transponder), you might need to upgrade to a Pi3 to free up a USB port for the second dongle. In the meantime all the latest software runs on the Pi2B - just a bit slower than it will on a Pi3. If you do decide to upgrade to a Pi3, remember that at the present time PilotAware only supports the Pi 3B NOT the 3B+ which needs a different setup.
Regards
Peter