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Hard to offer equipment advice without even a hint what your budget is. If you're looking to buy additional lights, a few cheap used strobes (e.g., Nikon SB-80 can be found on ebay for $$80-$110 per strobe) and some inexpensive radio triggers (e.g., used Cybersyncs) might be a good route. You'll need a couple of stands as well. And you'll need time to practice with them. That's assuming you already shoot competently in full manual mode and have at least a basic understanding of how to use off-camera flash. If you're contemplating trying a completely foreign setup at a wedding, that's just crazy, and you should follow Diggler's recommendation to use a decent flash bracket and go with that.
If you only have one body, you should get your hands on a second one. You probably have a friend with a 5Dc or 5DII who would let you borrow it for the weekend -- no need to buy a second body for one wedding. But you definitely want two bodies. Aside from the convenience of having two bodies with different lenses already mounted, you've got a backup in case of emergency.