If there is little interaction I would probably write the two scenes separately and then inter cut them. Other that that it depends on your writing style. For a complex scene I would at least work out high points that it will hit from start to finish, for something really complex perhaps getting most of it worked out in shorthand first.

If however you write when you are in the mood as I often do, planning isn't often a priority. In that case just go with your gut, do your best and be prepared to edit well.