[ScottC]

When I took delivery of my car, the dealer set up Mercedes Me apps on TWO different phones. Mine and my partner's. Each of us has different log-in credentials. They are tied to the same MMe account in the back end. And, indeed when I use the web to log in to my MMe account, I can see my partner at the bottom of the page as an additional user.
With this setup, each of us can use our own MMe app to add our own profile photos, which are then uploaded to the car.

I'm sorry I don't know the details of how to do this, since the dealer did it for us, but I am writing this to tell you that it CAN be done and advise you of the general setup needed.