Advice for event shoot
First time I saw your post I shied away from diving in because there are so many variables .. but what the heck.
The key difference as compared to your normal work is to be aware that you have to "catch" the moment. This is quite different to studio work where you have much more control - in most senses you have the chance to "create" the moment. Therefore the advice from chasg about keeping on your toes and looking all the time for interesting moments is absolutely correct.
Clients will overlook (in fact most of the time not even see) technical issues if you captured the moment - that's what they're looking for and that's what's memorable.
Take lots of photos and take them in raw - you're not in control of the lighting either - and you may have all sorts of strange colour effects to adjust later. Others may well say that you should sort out the white balance at the time and just take perfect shots. I'm going to say you just won't have time.
I'd take an f2.8 lens if you have one. Otherwise my choice would be to put the 24-105f4 on the 5D. It gives you more flexibility than the 17-40 and you don't want to be changing lenses. On a FF camera 24 is pretty wide. On the other hand, also on a FF camera I doubt 40mm will give you enough reach. (I'm saying this of course without knowing the size of the venue).
Put the flash on the 5D but try not to use it more than you have to, especially during sensitive parts of the ceremony, where you could be a distraction. Use a high ISO setting instead and (if necessary) remove the noise later with Noiseware or similar software. At other times of course use the flash to fill the photos.
My two cents worth on camera settings is that I start out in Tv mode and use 1/80 second. This is just fast enough to stop motion blur in settings like this where people aren't exactly running. 1/80 sec and 800 iso is generally enough to let me work without flash. (My main camera is a 1DIIN and most often 24-70 f2.8).
The dragon dance however will be faster. There I think you will simply have to use flash. In any case I doubt flash would cause offence at that point of the celebration.
One more thought. Approach the head of the temple and offer to take the photos. If you can get some sort of "nod and wink" as the "official" photographer it does make life easier in these days when everyone there will have a digital camera of some sort.
What I generally do with the second camera in situations like this is set it up on a tripod with a wide angle lens in a spot where it can catch as much of the "broad picture" as possible. I then leave my wife/assistant with that camera and a remote shutter release and instructions to press the button whenever anything interesting is happening. In this simple way I can mix wide, ambience shots with the tighter framed shots that I've taken myself. (Synchronise the time on both cameras and DPP will sort the shots from the two cameras together and then rename them as one continuous sequence.)
Finally, at the risk of telling you how to suck eggs, remember (as chasg says) that this is a ceremony and not a model shoot. Observe the necessary etiquette and preserve the dignity of the ceremony.
'The important thing is not to stop questioning.' Al Einstein
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