Player 1: Get your first portal on the wall panel directly in front of you at the beginning of the test chamber, and place your second portal on the wall to its immediate left. This’ll create a light bridge right across the water, allowing player 2 to perform their next task.
Player 2: Run down to the end of the light bridge and turn to your left. Fire your first portal on the wall straight ahead of you, then come back and stand on the permanent light bridge and replace player 1’s portal with your second one.
Player 1: Make your way through player 2’s new path and at the end of the light bridge, look for the wall panel just up ahead and to your right. Set a portal here and double-back on yourself back to the start of the permanent light bridge. Rest your second portal over the top of player 2’s portal, thus creating a new pathway.
Player 2: The final part is reasonable easy, place a portal at the end of the new light bridge, look up to your right and get an exit portal up here, step through and follow it all the way to the exit along with player 1.