On some camera lines (e.g., Olympus), and some cameras and lenses, image stabilization on each can work together to increase the stabilization. On my older Canon camera and lenses, the instructions always said to turn off IS on a tripod, and I did. I've found that on my Olympus setups it doesn't make a difference, so I don't turn it off. Which is a good thing, because it's hard to remember to do it every time.