Suggest doing this on
"Chapter 7: Alone" in the Wasteland. Here you'll get the best opportunity to get
two "Gunk Beasts" to charge into one another. In the Wasteland chapter you need to clear pools of Gunk to survive, once you've cleared the
third pool use this as the checkpoint to earn the achievement. As you continue on from pool three,
a herd of three Gunk Beasts will appear on top of the hill. Walk up to them and simply
get them to charge into one another and the achievement should pop. See the video for more detail.
After the Wastelands the next time you'll come across two Gunk Beasts is at the last boss. It can be tricky to do the achievement there as the boss can one shot them with his laser attack.
GTKrouwel83 mentions the
best method for the last boss, is to pull both leavers/chains, so the boss is kneeling and not firing lasers. Now you can lure both together without worry of them dying. If you have the "Power Pulse" upgrade unlocked you can stun the Gunk Beast with
+
to help line them up (credit ninjaraiden2003).