My friend and I did this in Chapter 3 Chamber 3, we failed to complete the 5 portals achievement and were goofing around and we tried this one.
Any level with a Light Bridge & Water/Acid/Goo whatever it is.
How to -
While they are on the bridge have them gesture above what looks to be water/acid (Valve calls it "Goo) while they are gesturing shoot the bridge to another location and out from under them, they will fall into the "Goo" to their death and you will receive an achievement.
Thats it, hopefully you are nice enough to let your partner get the achievement as well :P.