With typical double buffering you have two "windows". One is the display (what the user sees), and the other is the "back buffer" (what you're drawing to). When you swap, you're basically flipping them. So the previous display becomes the new back buffer and vice versa.
So no... you can't swap a portion of the display. It's all or nothing.
You can draw to only a portion of the back buffer, though. See glViewport.
I tried the glviewport before, the things seem scaled to the size, so i have to figure out a way to enlarge them back, but i think this will be even more costly compare with drawing whole thing. Well thanks for the reply though.