Let's say that the bike portion will take you six hours. You can make all six hours of fuel in two bottles. For example, if you have found (through testing in training) that two scoops of Perpetuem will provide an hour's fuel, put six scoops in and make a 3-hour bottle, fulfilling your hydration requirements with plain water (approx 20-28 fluid ounces/600-830 milliliters per hour) from another bottle or hydration system. Make two 6-scoop bottles and you've got your six hours worth of fuel taken care of.
Mix the bottles the night before the race and freeze them. That will keep them cold and palatable as they thaw during the bike portion of the race. If, however, the weather on race day is going to be very hot, or you don't have a way to keep the bottles cold until the bike leg, then try this method: mix and freeze one bottle as above, but in the other put only the six scoops of Perpetuem, and add no water.When you finish your first 3-hour bottle you will need to stop and take a few moments to add cold water to this second. It just takes a few seconds, but now you have a fresh, cold bottle of fuel for the second half of the bike ride. Using insulated bottles also helps, if you don't mind the slight extra weight.
One athlete who completed a hot-weather Ironman Australia used this method and wrote us saying, I did as you recommended - had the second bottle of dry Perpetuem powder in my bike Special Needs bag, and just added cool water. It only took 30 seconds or so and I was underway with a bidon of cold Perpetuem for the second half of the bike. Great suggestion, it works like a treat!
