It is fairly common. I have no idea percentage wise, or whether "most" people do it, but it is certainly fairly common.
Are you the one looking to be a second shooter? Or are you the one considering getting a second shooter?
If you are the one who wants to second-shoot, then there should be plenty of opportunities to do so. Ask around with friends and also try calling up advertised wedding photographers in your area. Don't do it for free, unless the person you are second shooting for is very well known and very good, and if you also really have zero portfolio to speak of. If they are less well known, and if you aren't completely desperate, then request some sort of fair compensation that takes into account that you are likely less experienced than they are. Also, you have to take into account that their reputation is more at risk than yours, and that they are spending money on advertising their business, etc., which you aren't (yet you're still reaping the benefits). I'd say about 25-35% of revenue is fair for a typical second shooter. OR payment by the photo - the more of the second shooter's photos end up going to the client, the more they get paid.
If you're the one looking for second shooters, then the above still applies: Pay them fairly based on what they contribute, and do your best to actually teach them whatever you can, and they will say good things about you and guarantee a steady stream of other 2nd shooters for your business whenever you should need them.
In either case, second shooting is a fairly low-risk option for everybody involved, and you shouldn't freak out too much about it either way. Just give it a try and see if it works out.
Two shooters gives you a huge advantage because you can get two perspectives. This is ideal for weddings since it is the joining of two people and you can capture the event from the bride and groom perspective instead of having to choose one over the other, always go bride in the position. And you have back up and will usually get different shots that one or the other of you will miss due to just not being in the best spot.
Last wedding I did I had a second shooter and he managed to get the one decent picture of the best man. Every picture I took the guy had some weird look on his face. And one wedding I was the second shooter for I was the only one that managed to get decent photos of the kiss.
99% of the time I use my second shooter. I use the same one. Two perspectives is better then one, and I can't be in two places at one time. I go front while she stays back until toward the end and then I finish it out.
Im a second and Ive been doing this for a couple of months now. The photog I work with has two bodies on him the whole time, a 85 and a 16-35. I usually shoot 70-200 for the long shots and I try to capture a different angle. just like in film, you should think of the shot in 3 separate views... the distant shot, the medium and then the close up. That way you get a great selection and best composition.
I use two shooters. It just makes everything a little more streamline. As Kathy pointed out, I can't be in two places at once. As Joe pointed out, two perspectives are better than one. She'll catch things that I don't notice, and vice-versa. It works.