Same answer, basically, that I gave in a recent thread, but... if no one is depending on you... go shoot with whatever you have and do what you want, just do not interfere with the main photographer, but if there are expectations of you... the minimums are:
1. Have a STRONG understanding of the basics of photography. If you cannot talk about aperture, ISO, shutter speeds, the Zone System and have not mastered your own camera at the advanced levels... you should not be considering doing this.
2. Have a STRONG understanding of the basics of lighting. That means off and on camera lighting!
3. Equipment can be summarized in 3 points:
A. Quality FAST glass. If it cannot do constant F/2.8 or wider.. its not wedding ready.
B. LOW noise at higher ISO settings. If you cannot get clean ISO 800 shots from your camera, its not wedding ready.
C. Have backups of EVERYTHING, that includes your cameras. I carry a minimum of *3* of everything to each and every wedding I do as a second shooter.
My cameras are a D700, D200 backup and a Nikon F2a and even an E8800 for really those "just in case" moments.
My lenses are all pro level glass and go from 15mm to 200mm zooms, and I have 15mm F/2.8 fisheye, 30mm F/1.4, 50mm F/1.4, 85mm F/1.4 and 105mm F/2.8 macro primes as part of my kit. My zooms are 18-50, 24-70, 70-200's all do constant F/2.8 apertures.
I carry 3 studio heads and 6 speedlights with me, and a minimum of 3 sets of Energizer 2650 NiMH batteries per flash are recharged the night before.
The only thing that I do not have backups for are my Pocket Wizards, but I do carry a backup wireless triggering solution if needed.
I would by now consider myself an advanced 2nd shooter, well ready to shoot weddings on my own, but honestly... I loved being a VOL (Voice Operated Light stand) for 3-4 weddings to see how and what the main photographer worked, without having the pressure of needing to put out pictures for the client. Then I became a 2nd without the pressures of needing to "produce". Anything I shot was 100% for me, the client never saw ones... unless the main and I wanted them to. Now, I am a contributing photographer, after about 10 events with my mentor... and loving it.