What I have generally done in a situation like this is to create two separate listings. In the Midwest where I have done most of my work, it is rather common to have a large bridge over a main waterway and then a smaller bridge over an old channel nearby. The NBI tends to treat the overflow bridges as separate structures, and I have followed suit on here.
As an example...
Main Bridge (end of overflow bridge visible in Photograph #3):
Overflow Bridge (sorry, no photographs):
I took some pictures of that other section, and was wondering about that myself. Is it part of the bridge, and should pictures of it be included here?
Traveling west on Washington State Highway 4 in the Google Street View section, there appears to be another bridge or elevated structure that is part of the same old alignment.