1. According to the following link page 4, I believe it is the lower that is opened. View the diagram to be sure--referred to as test cock # 1.

http://lawn-aerator-attachment.com/Winterize%20Backflow%20Prevention%20Assembly%20and%20Sprinkler%20Information.pdf

2. You use both! Read through this...

Winterization Steps:
-Turn off main water to the yard
-Seal/tag off all closed valves
-Open any low points/drain valves to allow sufficient time for drains to fill (gravity drain) *if they have them
-Close low point drains *if any
-Connect air compressor to blow out connection
-Program each zone for a minimum of 2 minutes and a maximum of 5 minutes
-Turn on zone 1 to relieve pressure on system
-Manually open zone at furthest point of main line with bleeder screw/solenoid in valve box
-Open hose bib or compressor connection valve
-Turn on air compressor, SLOWLY open valve on the compressor. The compressor does not need to be fully open to achieve this. Run zone until only mist comes out of the heads
-When misting is visible, open next zone working your way back to the main supply. NOTE: You are able to blow out multiple zones at the same time ONLY if all heads are on all zones continuously at full pressure (i.e. you are able to push the heads down into the ground with minimal force.)
-Continue running through all valves manually until misting is seen on each of the zone heads. NOTE: Don't allow air pressure to build between switching of valves. Ensure that there is always one zone open at all times.
-Once all valves have been done manually, repeat the process again for all zones using the controller
-When finished running through all zones with the controller, TURN OFF the compressor. Relieve air on compressor by opening ball valve not connected to the hose
-Adjust program to 0% season adjustment
-Turn controller to the OFF position, DO NOT unplug controller-remove main water control wire
-Contact appropriate personnel to remove water meter, if needed

This guy actually uses the top one.

https://www.youtube.com/watch?v=LEL33cc7tnI

Maybe check with Febco and ask.

Thanks for the reply. The Youtube is the same as what manufacture support's send me in email. I saw it before. By reading parts instruction, it seems that air compressor connects to anywhere but test cocks! I met with a commercial property irrigation guy and he told me hook up with the drainage faucet BEFORE back-flow breaker.

In your paragraph, you said that 'Connect air compressor to blow out connection', where is the 'blow out connection'?

Thanks again!