Do you know for sure that you can use a flash during the ceremony? Do you plan on taking solo portraits of the bride? (I certainly hope so!).

I should point out that you have a 1.6x crop body and are proposing another 1.6x crop body, and you have all f/2.8 maximum aperture lenses. That leaves you a bit lacking in the thin DoF and low light department. I'd consider a faster prime (50mm f/1.4, although it's a bit soft wide open, or the 85mm f/1.8, which is an excellent portrait lens).

If you plan to shoot with both bodies (i.e. 17-55mm on one and 70-200mm on the other), you'll probably want a Speedlite on both of them.