Your students just finished a two person scene. Now it's your job as their teacher to check in with them. Asking surface level questions doesn't help nearly as much as specific questions to get them thinking about what they just did and what that means for the specific objective or goal you are working on together.
Here are some debriefing questions you can ask after running your improv exercises:
Did you find that exercise difficult/easy and if so why?
What about the exercise did you like/dislike?
When did you feel lost/tense/struggling, if at all?
What thoughts did you notice, if any during the process?
Keep in mind there are no "right" answers to these questions. We are there to facilitate growth and learning. By asking questions, the student has to dig deeper to figure out their own answer. This is where growth occurs.
Next time you are in class give some of these questions a try and see how it goes. After you do, debrief on your own experience and see what you too can learn!