Hmm... a bit of a bizarre request from the B&G if you ask me... one of two things is going on here.. either there's a miscommunication and they don't actually want you to be the "second shooter" and just want to make sure they have people other than the pro taking pictures from a guest's point of view, OR they don't have any confidence that the pro will be able to capture the pictures they want.
The first scenario makes more sense, and may have used the term "second shooter" in a less literal sense, whereas you're interpreting their request very literally as "be the second pro photographer".
I guess the second scenario could happen, in which case we're left wondering why exactly they're paying a pro photographer to take pictures they think they won't even like.
Either way, as many others have already expressed, the pro photographer will not allow you to take pictures of scenes that he or she has arranged, and will be very annoyed if you try to shadow him or her all day and will probably ask you to leave.
Many, if not most, professional photographers require B&Gs to sign a contract that explicitly prohibits having a "second shooter". Additionally, contracts often contain clauses that prohibit allowing guests to use their cameras during the formal shots, etc. I certainly require my clients to sign such a contract. Of course, on the other hand, guests are certainly expected to take pictures of the ceremony while they are seated, and of the reception while they are mingling with the other guests. That's what I suggest you do. You will be able to take lots of great pictures in this capacity. But taking time with the B&G to do your own portraits with them will be seen by the pro as "stealing" his or her time with them. It would also be very bad for you to be walking around during the ceremony. The pro should be the only one out of their seat.
Hope this helps! You should talk with the B&G to very clearly articulate your expectations and let them know that while you will do your best to take photos, you won't have the same level of access as the pro and therefore they should not expect professional-level pictures.