After placing the first portal on the white wall panel (on the left side of the chamber), turn around and look up and to your right. The second portal should go where the bridge meets the wall. Now walk through the portals to the Companion Cube.
Before picking up the Cube, turn to your right and you should spot a white wall panel on the right-hand wall. Reset the first portal so it’s coming through here and pick up the Cube.
Take the Cube back through the portal behind you and set it down in front of the laser (so it's also directly above the red button below). This will cause the elevator up ahead to descend.
Drop off the light bridge and run over to the elevator, now turn around and place the first portal right beside the second one, causing the elevator to rise, allowing you to exit the test and start the next - brief - chapter!