I was recently asked to be the secondary photog at a friend's wedding, mostly in charge of the guest candids and pre-wedding "getting ready" shots. We all had a lot of fun.
But then, the primary photog was a fairly young girl (ok, Im not an old hag or anything, just def. over 20
)with a Canon XTi & kit lens, her first DSLR, I think.
So, she wasn't one of those photogs who thought I was stepping on her toes or anything.
I was also very careful NOT to be in her way whatsoever.
If you decide to do this, take both cameras, you could change your mind!