Since it's a bit convoluted, here is a step by step of the split screen method:
Requirements- MAIN gamertag (the one you want to get the achievement with).
- 2nd gamertag (Gold not needed) that is friends with your MAIN gamertag.
- 2x Xbox controllers.
Method1. Sign in as your 2nd gamertag, load up Garden Warfare 2, create an EA account and complete the short tutorial.
2. Now you should be in the backyard as a Sunflower. Walk over to and press
on the Split Screen menu (it looks like two TV's stacked on top of each other) and press
on your second controller.
3. Select your MAIN gamertag and you'll hit a load screen.
4. If you did it right, your 2nd gamertag will be top and your MAIN gamertag will be bottom of the split screen.
5. On your MAIN (bottom screen) character, switch to a zombie character in the Character Select booth, then walk over to your 2nd (top screen) character and kill it.
Achievement UnlockedYou will also unlock the achievement: