Ardupilot GPS1 & GPS2

I am using both Ready to sky GPS and Piksi Multi GPS. How can I understand in mission planner that which one is GPS1 and which one is GPS2. Also, is it necessary to use primary Ready to sky GPS if I use Piksi Multi GPS?

You should see in Messages which GPS is detected as 0 and 1
I would set the Piksi as primary and set GPS_AUTO_SWITCH,4

4:Use primary if 3D fix or better, will revert to 'UseBest' behaviour if 3D fix is lost on primary

Also set your GPS_GNSS_MODE and GPS_GNSS_MODE2 so you get most reliable/lowest GPS HDOP.
Particularly that Ready To Sky unit will be overwhelmed by the number of constellations.
Check in logs for GPS HDOP
and GPA Delta (should be a straight line at 200ms)