Flying with another Captain is definetely a learning experience.
If in the right seat, as difficult as it may appear, I try to make myself a F/O in every sense. Trying to go back to the feeling I had when that was my role and I did my best for my Captain.
Yes, most of the times I would use a different approach on many issues during the flying day but that goes with the Captain personality.
Concentrating on being a good F/O makes it an interesting day anyway.
Also if small details only, I always discover something that, on my next flight, will help to make my F/O feel more confident and at ease.