Carefully separate the two pieces by hand.
The two shoulder buttons are made up of three pieces -- the plastic button, a pin, and a spring. If they accidentally pop out while you are working, study the picture and make sure you put the spring in the correct position.
When reassembling the cover, make sure that the power switch cover on the bottom case is in the off (down) position. Otherwise, the power switch cover can break the very fragile power switch upon reassembly.