I think that since you are secondary photographer, you can have a bit more freedom. Anyway wedding photograhy has a lot of styles. If you are going for wedding PJ style, I think your 28-75 and 135/2L should be able to do the trick (though i would prefer something wider). If you are planning to help out the primary photographer in taking the traditional type of photos (like event-based: Photos of the family at the wedding dinner table..etc), you will NEED a wide anlge as the 28-75 is too short and a flash. The so-called money shots that the primary photographer has to take require a wider focal length because there are moments where you need to capture the picture and you dont have chance to back out. And the flash is needed too because as mentioned, you dont have time to bring in lights. In a wedding, you want to be as distruptive as possible and thus, a wide angle and flash is the way to go. On a 1.6x body, this would mean a 17-40L and 580EX for example. Personally, i found the 17-40L to be one of the best tools for this kind of work (shooting with a rebel). I cannot stress how important it is to have a wide angle lens and I personally do not think a 10-22 +28-75 is a good combination for this kind of work. Why? because you will be working within the 17-50 range a lot, and you dont have to time to switch lenses in and out.
So if you plan to shoot solo next time or are going to help the primary photographer with the money shots, go get the 17-40L + flash.
So if you plan to shoot solo next time or are going to help the primary photographer with the money shots, go get the 17-40L + flash.