Len i'm hoping Andy EOS1 NRS or newbloke see's this as I think thay may have shot most weddings here (not 100% sure though but I know EOS aka donkey boy has shot a ton).

Paul has done some nice work and knows how to cut the mustard.
I have shot 4 weddings ( I loathe shooting them) but 75% of my paid work is portraiture.

I may be pulled up on this but this was my own approach with lenses. The lens span you have should be fine. I used three lenses with two camera bodies. 16-35, 24-70 and a 70-200 all fast lenses. WB set to daylight until I moved into the church. Don't be afraid to shoot wide open and don't be put off by slower shutter speeds. Also do not be put off by shooting with a higher ISO...some wedding guys shoot at 3200 and have superb results. If you have auto bracketing then set a stop each way if shooting in jpeg. If you are gonna shoot RAW then no need to bracket. Considering you are not the main shooter you should be able to relax and enjoy this shooting day. Stay out of the way of the main shooter..I REPEAT STAY OUT OF THE WAY OF THE MAIN SHOOTER (said with a smile) . Look for good light. Light is what makes an image in my personal opinion. oooops!!! Hold on a moment...i'm going off on a tangent sorry mate. You were looking only for lens advice..my apologies. The two lenses you have should be fine. Remember to charge your battery and have a great time.

(I get carried away sometimes..hey! i love shooting! )