When there is more than one device of the same type (like a GPS) on the NMEA 2000 network, the NGW-1 will use the data from the highest priority (lowest source address). If the device you'd rather use as a secondary Talker for the NGW-1 claims a lower source address then unfortunately there is no easy way to change this. The first step would be to try changing the device instance of the secondary Talker to 'instance 1' (leaving the primary at 'instance 0').