When it comes to weddings, I really prefer shooting in manual exposure mode. Some other people will prefer aperture-priority or even program mode, and that's okay for them. When you're in lower lighting and at various times getting spotlights or windows (especially stained glass windows, which will catch you by surprise), you need to be really careful about exposure. That's why I prefer manual.
Anyway, the reason for considering the exposure mode with lenses is with constant-aperture vs. variable-aperture lenses. You cited the 28-70/2.8-5.6 lens, which might be the f/3.5-5.6 model...? Sony makes a constant f/2.8 lens that's 28-75mm and another lens that's 24-70mm. When you start zooming in-and-out, your exposure will shift on you, which can be frustrating.
This is why I prefer constant aperture lenses. They're more expensive. Definitely. But if you're going to get serious about wedding photography, you might want consider that. Weddings generate income, which pay for the lenses.
In terms of focal length ranges, I generally use two cameras with a 70-200mm on one, and 24-105mm on the other during the ceremony. For the reception, I switch to 17-40mm and 24-105mm as a starting point (but I'll swap lenses as needed). And this is all for full-frame angles-of-view.