It depends! If you're touching the same area in the same file -- e.g., if the themes are close to each other alphabetically, then there's a likely chance of conflict. If you're touching different areas, then it likely won't cause a problem.

The quickest way around it is for us to be quick about committing stuff, before the patches start stepping on each other :x

(Yup, I got it, but I meant to look something up before replying. I should go do so now!)