I have been in many discussions regarding a topic similar to this at where I work. It has to do with the value of a just out of college type person versus maybe someone who didnt really go to a 4 year college but has 10 to 15 years experience instead. Dont get me wrong in that college is a GREAT thing and education of course is good. There are however various times where experience makes a difference and maybe has a bigger influence on decisions.
An excellent example would be if you were to have a special kind of surgery that isnt done very much. Would you go to the young doctor that just got his licenses and the ink is still wet or would you go to a doctor that has been around for 15 to 20 years and has done this surgery 75 times already?
Professional videography is sort of the same thing. There are special little things that one develops along the way that will help make the end result turn out that much better. Obviously you have the education. In fact you may have more that a lot of us on this site so I have to think that you have the basics down and understand how everything works.
Do I think you can do it?
I think you can.
Now does the bride think that you can do it?
Keep in mind that she just might be a little more nervous and apprehensive because after all she is getting married for the first time and she planning the biggest day of her life. All she wants is to make sure that everything goes right because remember there is only the one ceremony and you only have one crack at this.
As far as your question, I think it is totally up to you. How confident and comfortable do you feel? Everybody here pretty much started out the same way with doing their first 2 or 3 wedding jobs for nothing. Usually its family relation or close friends because if something did go wrong and hopefully it wasnt a total loss, the couple would maybe be a little more forgiving and wouldnt be to upset. The fact that they didnt have to pay anything will help too. Later on down the road when you start taking on jobs and you are starting to charge big money, you will find that couples will not stand for screw ups and boo boos.
IMO… trying to work around another videographer at the same wedding will be clumsy not to mention awkward and crowded. It will also be a distraction because you will have 4 to 6 cameras on tripods all over the place and the two of you would be fighting for the money shots. I would either take this brides offer of doing it or I would choose not to double team it <BUT> I would instead watch what the videographer does during the ceremony and learn from that. You could maybe pick up a few things as far as how he/she planned the shots and operated during the ceremony.
As far as a little piece of advice to leave with you… you have to make sure you get the actual ceremony done right! After that just shoot b-roll until all of your batteries run dead. Once you have everything on tape the pressure is off. The creative editing is a little less stressful because can always change… re-do… or undo until you get it the way you want it. That is where you put the show together.
Good luck and may your camera always stay focused!