Here's an easy way to get both
with a minimum amount of effort:
- Play vs mode with two controllers: your profile as Infected and the second controller be the Survivors. Regardless of the map, use the Survivor to kill two of the other survivors.
- Once two of them are dead, spawn as the Jockey. Jump on the AI Survivor and ride him until he's downed*.
- Move your Jockey out of the downed AI's sight and bring the player controlled Survivor to the Jockey. This is so the downed AI cannot kill the Jockey.
- Use a health pack on your player controlled Survivor if he's damaged to ensure he's got plenty of health.
- Hop on the player controlled Survivor and ride him until he dies. You'll get both achievements.
*I didn't have any problems with getting out of the AI's sight in time, but if you wanted to play it safe, down the AI character twice first, so when the Jockey takes him down, he dies.
If you've got a friend on XBL, have them join you as a Spitter and you can easily knock out
at the same time.