Actually, you miss the point. Switching bikes has nothing to do with what is optimal for either rider.
This underscores a basic tenent that transcends cycling. If you don't know, it doesn't matter.
PS: putting another spin on it...you are optimizing something. It just isn't comfort or speed for either rider...one fits perhaps better than the other.
What you are optimizing is...the economy of two riders sharing the same bike. Nothing wrong with that at all. Road bikes are expensive. If you can share a bike and both are happy with the arrangement...all that matters really.