My usual strategy at that point is to look at whatever info is available on their character, brainstorm 2-3 locations our characters could conceivably overlap, and offer all of them as suggestions.
Then once we’ve picked a spot and the scene starts, it’s just a matter of picking something that brings the characters into contact ASAP. Either that’s posting a starter the other person can directly respond to, or responding to their starter in a way that gives them something to work on. A scene can still be shaped a couple poses in - you’ve almost always got more time to figure out what you’re doing than the 30 seconds before you have to answer that question in your scenario. You’ve just got to break it down into smaller steps.