Already looking good so I wouldn't stress out too much. Keep practicing, and try to be as prepared as possible so you don't get stressed nearer the time.
I'm not a wedding tog but have shot 1 fully, and assisted at several for friends, but I will try to give my 2pence
* Do I need two flashes?
I don't think you need but 2 flashes will give redundancy. It all depends what lenses you have as well. Say you shoot like An Exception with a 35mm on one camera and an 85mm on another then a flash on each makes sense, if you with a 24-70mm on one and the 70-200mm on the other it is unlikely you want one on the 70-200.
It is somewhat similar argument to needing 2 bodies, you don't really but you do need backup and it does make life easier. I only have 1 flash but have a few events booked for the rest of the year so I might buy another flash.
*Do I need to learn off-camera flash work, to take control of bad lighting, say in the pub?
I would say no, unless you want to. If you setup a static light stand and things then it only helps when you pose people and put them in the right place. I prefer reportage/photojournalist style so don't really like to pose people. Also, it is very hard to do well. I see a lot of people with loads of lighting gear and off camera flashes here there and everywhere but the results look very fake. I like natural light and try to maximize that, and find that simple bounced flash can work very well. But I am not a flash person, others will have different opinions.
*What do you guys use to walk round with two heavy cameras all day?
Someone will mention black rapid. The market is now fully of alternative straps and clips with loads of cheap Chinese stuff that is made as good if not better than black rapid for a fraction of the cost. I order 5-6 off amazon to test and kept the best. All were fine TBH.
What combination of lenses 3-4 you suggest I have on each for each part of the day?
What communication do I need to have with the venue? Should I scout out the church first, also the pub?
*How many good quality photos do you expect to hand the couple after your day's work?
Good question. The typically issue I find is you have 4-6 great photos, 10-30 very good, 50-100 good and can end up with hundreds ok acceptable. It all depends how much of the acceptable photos you want to dilute the set down with. Furthermore, many of the acceptable ones just ends up being the stand kind of photos you alway end up needing (photo of ring, wedding shoes/dress, generic makeup shot, kids being cute, church, flower arrangement, group photos, cake cutting, cake knife). These are kind of needed. (Or expected) but don't make fascinating photos IMO. So you will end up with a few hundred anyway.
What do you do about crappy weather? It'll likely be peeing it down and cold, so no outdoor shots, but is there anything to think about there?
* cloudy rainy weather is just God's light tent, perfect conditions for shooting outside. Don't let the rains top you, the best wedding photos I've seen were done in the rain, not the clichéd sunset photos. On my sister's wedding it was pouring down all day, much like Hollywood set -just drenching rain. The photogrpaher was well prepared with big pink Wellies and a large umbrella. Plots of photos in the rain, standing in puddles, getting the dress muddy, standing in Assam with the Wellies on. I don't have these photos so I can't show them but I'm sure google must have lots for inspiration.
Edit:
http://kristenhoneycutt.com/blog/wp-content/uploads/2011/05/Jessica-Jeff-Wedding-0347.jpg
http://www.lwdbridal.com/wp-content/uploads/2012/06/rain-on-wedding-day.jpg
http://dpnak.com/wp-content/uploads/2012/10/kyrie-wedding-watermarked-blog24.jpg
http://ellybevents.com/blog/wp-content/uploads/2013/01/rain-delsol.jpeg
http://isgphotography.files.wordpress.com/2011/08/wedding-in-the-rain.jpg
http://www.styleinspirationanddesig...n-the-rain-Lisa-Sammons-Events-Weddings-1.jpg
http://ppcdn.500px.org/43797738/58059b49483b6d125375474703f2552a2fb18ea3/4.jpg
http://www.bridalguide.com/sites/de...ain-wedding-photo-unplugged-photography-2.jpg
https://beebeyecrew.files.wordpress.com/2012/03/hoffer-rain_large.jpg
Going by the above I would be praying for rain if I was you!