You can gain this achievement after turning on the blue Gel at the pumping station for the first time.
Once you turn on the Gel follow the path and you will hear Cave Johnson talking about an offer from the zoo, after this you will have to long jump on top of a closed test room before you jump down look out to the water on your left and fire a portal at the base of the tower, double back to the start in order to place your second portal.
Once over follow the tower up and then the long path on the catwalks till you reach a sealed door, turn the leaver for the achievement to pop.
You can reach this specific area a bit quicker by typing "Changelevel sp_a2_past_power"