If you really have to share an altar, then at least have your own altar cloth, candles and tools. Ideally, maybe have two lightweight ones for separate use. I think that you can banish energies from a shared altar but of course, in your mind, you will know that its not entirely yours. So you need to think whether knowing that would affect your workings.
Oooh, I would love to have a ritual room just for that (IMG:
style_emoticons/default/sun.gif)