If you are shooting the first dance you aren't going to be able to avoid using flash, unless the first dance is outdoors and is earlier in the evening as sometimes happens now. Flash can also be useful during getting ready as well. Obviously you shouldn't use it during the ceremony as its too distracting and depending on your style you may not require flash for portraits. A small reflector can be useful for portraits.
Not really that familiar with Leica equipment but the Q2 has what a 28mm lens? So that gives you a 28mm and an 85 that should be fine for most things. Tim mentioned above that shooting wide open only works for photos of single people because of d.o.f that isn't strictly true. I am guessing he is referring to group photos. I actually love group shots taken with longer focal length lenses wide open and often use my 85mm f/1.4 and 135mm f/1.8 for this when I have the room. As long as you keep them all on the same focal plane it isn't a problem and the background separation and compression can work really well. Although recently I have switched to using a 50mm f/1.2 for this as using this wide open can provide similar separation while losing compression but with it being a 50mm I don't need as much room. You will need to keep any groups in the centre of the frame if you are using the 28mm though because of distortion at the edges so allow room and crop later.
Do they both have dual card slots? Not sure with Leica. If not to limit any potential loss if they don't have dual card slots you may need to go old school and use multiple smaller cards so that if a card corrupts you are only losing a small percentage of images.
There has been literally thousands of threads on here before about people asking the same question so a quick forum search should generate lots of info.
Things that commonly come up in these sort of threads are things like:
You should have a shot list (this is a load of complete nonsense, lists are a pain in the bum and end up causing you lost time which you won't have)
You should have public liability insurance (yes, you should in the unlikely event that someone trips over your camera bag etc. you want to be protected, equipment insurance should be a given too it wouldn't be the first time a wedding photographer has had equipment stolen at a wedding)
You need to manage the couples expectations. (This is an important one, you have not said why you have been asked to photograph this wedding. Is it for a friend? A family member? Is it the result of you putting yourself out there looking for wedding work? Regardless being your first wedding results likely won't be great, I know my first one certainly wasn't)
Stick to your personal strengths. (What sort of photography do you normally specialise in? As a wedding photographer you need to be be a portrait photographer, a product photographer, a landscape photographer, a journalist and an agony aunt all rolled into one. Concentrate on doing the stuff you already do well, with the rest learn as best as you can before the wedding.)
Know how to manage people. (This can be a tough one for some photographers as a lot of photographers tend to be introverts. You aren't going to be able to hide behind a camera at a wedding. Even those that are purely "documentary style" need to know how to manage people, be confident and sometimes forceful.)
There are lots of things that can go wrong at a wedding, you can't really do much about that, but do what you can to mitigate these. Time is a big one, set a timeline with the couple, always allow more time than is needed for everything as they will always run late at some point during the day which screws the timeline. Make sure you create multiple back ups of everything. Have a plan of what to do if something goes wrong for example say it rains all day, is your equipment going to be good enough to shoot all of the portraits indoors without using flash?
Wedding photography isn't as difficult as people like to make out as long as you are aware of the pitfalls and have a plan to deal with them. Most issues are caused by not being able to gel with the couple. Even if it's someone you know very well and have been friends with for years can turn into a completely different person on their wedding day. If you are able to get the relationship right with the couple it can solve a lot of problems.
Don't let venue staff and other vendors interfere. At times you need to be able to manipulate them or outright shout them down it isn't just the couple and guests at the wedding that you need to manage.
Only deliver your best work to the couple. Even if you have a set of amazing images their entire opinion of how things went and how good a job you did will be based on the weakest image you provide. It can be tempting to give them images that are technically weak but you think may provide some emotional attachment for them, don't do this!