Hi Dips,
I'm not all that familiar wedding photography. Here are
some pictures I short at a friend's Indian-style wedding. These were shot with an old Minolta A1 bridge camera that I've since replaced with a Pentax SLR. One good thing about Indian wedding photography is that no one tends to wear black (and white isn't all that common either), so you can get away with the smaller amount of dynamic range you get when you boost the ISO for low-light shots.
Two cameras are a must, yes.
For low-light photography you'll want as fast a lens as you can get. I don't think the Tamron 18-250mm you can considering is fast enough. Instead, I'd recommend going with the DA* f/2.8 zooms: 16-50mm and 50-135mm combo (one on each body). You could also go with the Tamron 17-50mm f/2.8 lens, if you wanted to save some money and the weather sealing wasn't important to you. You won't need anything longer than 135mm unless you are very far away from the action, or there is a huge crowd that you need to shoot over (which, mind you, can happen if the whole extended family comes to the wedding
, so bring a step ladder to the wedding ). You might also consider the various Pentax prime lenses, but you don't want to be caught changing lenses and miss the critical moment. Better to have slightly less high-quality pictures from a good zoom lens, than to miss a great candid photo opportunity because you had the wrong prime lens attached. That said, maybe you want to get one really, really (!) good prime lenses like the 77mm f/1.8 for the controlled portraits type shots.
It may seem a bit counter intuitive, but you might think about getting some neutral density filters. A wedding usually has some kind of agni-hotra yajna (fire sacrifice) and you can get some good long expose dramatic fire shots that a less skilled photographer will not be able to capture.
Oh yeah, and get a really good flash. Or better yet: several flashes that you can set up around the scene and remotely trigger. Or maybe that is getting too complex for you, if you are just starting out?