The 2nd day will likely take just as long as starting from scratch. I'd guess that since what you're going for is a specific temperature and time enough to break down the collagen, breaking up the cook won't buy you any time, and as Master stated, might mess up the texture of the final product. What you could do is finish them on the first day and warm them on day 2. This could be done on a grill or in the oven, and will save you some time as the breakdown of the connective tissue will have already taken place. This might actually enhance the texture, as gelatin, once cooled, requires a higher temperature but less time to melt than that required to break it down in the first place.