How to make HDOP2 data primary on Tower

Hi all;
I just bought a pixhawk 2 and begin to setup it.
I have a pixhawk 2 and i just connected to existing gps to gps 2 connection not to cut the gps 1 cable. when i connected the telemetry via Tower i can only see the hdop on the tower screen thus gps shown as “no fix” (because there is no gps in GPS1 connection) even hdop2 is 1.1. As a result of this i can not use tower properly with gps2. How can i set hdop2 as main gps?
thanks in advance.