Achievement Name - "Let's Go VS Force!"
Description - "Merge two VS units"
All credit for this solution goes to GameFaqs, I'm just making the info available on this site. (http://www.gamefaqs.com/boards/958322-lost-planet-2/54776519
Your gonna need at least 2 human players for this achievement. Start a campaign on episode 4-3, difficulty does not matter. The first VS you want, the GAN-34 Granseed, is above the first data point near where player one spawns. Another, if need be, is located on the far left side of the beach near some bunkers on a cliff. You must keep either one of these VSs functional and take it to the next area in order to perform the fusion. In the next area, after taking down the first electric barrier, stay left until you reach a cave. There you will find the second VS, the GAB-25M Cakti. To perform the fusion, man both the VSs and have both players press L3 and R3. With both anolog sticks pressed, move L3 to the left and R3 to the right. This should cause the Granseed to fly into the air and some hinges to come up on the Cakti.If done right, the option to merge should pop-up. Press "B" and after a little cut-scene it should be yours!
EDIT- The VS merge itself CAN be done in split screen, even if you're one person juggling controllers. However, you will not receive the achievement, only playing online will unlock the achievement.