In retrospect, I guess it was the ease of framing both captures sequentially in real time that I found motivating.
For some reason I've never really gotten excited about combining in post, but in this case it was kind of cool setting it up and getting the shots perfect with everything happening at once and to immediately see the result. I guess I may be suffering from the general societal malaise of prefering instant gratification.
Since the initial excitedment has now died down a bit, I agree I probably could have framed them pretty close to this and combined them later.