It uses any external device in preference to the internal one.
How "sticky" is that preference? There must also be some sort of hysteresis so it can revert to the external one if that comes back online.
I've found the bluetooth link to be very dependent on the GPS being well charged, versus the distance between the two units. So when the battery runs down (to a point which is dependent on the connection distance) there will be a point in time when the thing basically becomes unusable, but this point can't be determined readily, it appears. Then at some point the Ipad will try using its internal GPS, which I find highly flakey during flight. Whereas with the NMEA serial data used on all other portable devices the app can immediately tell if the data is rubbish - within about a second or two.
The inability to explicitly configure which GPS to use is
really stupid.
It may be worth 'feeding back' to Memory Map that this would be a good idea.
I have emailed them many times with no response, nor have I ever come across anybody who managed to establish communication with them.
TBH the moment Oziexplorer comes out for IOS, MM will be history. The only reason anybody in GA uses MM is because the CAA charts come in its format, but it doesn't take long for somebody out there to scan them in into a generic graphic file...
I do have the IOS app for the Dual GPS, which shows the sat data. It's slick. Oddly enough it won't compute a fix with less than about 8 "green" satellites though.