Once you unlock the second portal gun, find a place with portal-able surfaces above and below you.
Look straight up and place one color above you. Don't take any steps! Look directly below you and place the other color portal. After 6 or so minutes you should have unlocked the achievement.
Alos, make sure Chell isn't clipping the floor as she's falling through. One easy way is to place one of the portals at a 45-90 degree angle so as you fall, the portals will readjust your positioning through each one keeping you centered instead of slightly drifting to one side.