If you are going to carry spare parts etc strapped to your frame or bashplate or whatever, don't use cable ties. To be honest this is so obvious that I would be embarrassed to have learned this the hard way even if I hadn't have been advised about this first, which I had.
Cable ties work great for tidying loose cables together. In any application where they're under stress they will eventually slacken off and then break. If you're attaching relatively heavy items to your bike like sprockets, and then riding on bumpy non-tarmaced roads, this will happen quite quickly, even if you use multiple cable ties. Instead use wire (and pliars). Fencing wire was suggested to me, and you'd think lockwire would be a good bet as well. The added bonus is this makes things marginally more difficult to remove, and therefore steal.
Re: doubling up your control cables. If you fit new genuine cables before you go you should get a very long life out of them, far longer than the average trip. Cheaper and less hassle to just carry a repair kit, or (just a couple of pushbike brake cables and some solderless nipples, which you can even make yourself if you're that way inclined and too cheap to buy for about a pound each). If you get one of those kits that come in the little round tins that fit in your jacket pocket, you should probably have room to stick in a chain split link or two.