I believe that when you plug the external antenna in, it switches off the internal antenna. I'm not 100% sure of this - only about 95% sure.
Antennas on GPSRs require power to function, hence the reason that only one works at any given time.
FWIW, if the GPSR on your moto is mounted reasonably level (front to back) and nothing else on the motorcycle is obscuring the antenna's view of the sky, you really don't need an external antenna. External antennas are not 'more powerful' or 'better' than the antennas that are built into the GPSR - you only need to use an external antenna if something (e.g. a metallic windshield) is blocking the internal antenna's view of the satellites, or, if the GPSR is mounted on the moto in such a way that it is substantially off level (tilted well back, for example, so you can view the screen more easily).
Michael