I think i just found the solution..
1) connect the external display (both are working), and make sure the external is the primary display.
2) use a magnet to shut the internal display off (only the external is working by now).
3) lift up the magnet and quickly close the lid.
I succeed using the clamshell mode using this method. Hope to hear good news from you all.
Macbook Air 13inch 2011
OS X 10.8.2