You can put another card 2 slots below the video card... as long as it isn't another video card with a Zalman cooler.
Zalman coolers have a rised back plate as well as the fan that sticks out in front, so if you have two video cards, you'll need them to be 3 slots apart.
---VIDEO CARD---
---EMPTY SLOT---
---CARD---
---VIDEO CARD---
You get it?