Using a warm/hot bottle is a trick to stay warm, yes....but I have some reservations...
Before I go into my post, I'll start by saying that I haven't done the math for it all, so I dont know exactly how it works out weight-wise.
Its true that platypus bottles do not weigh much. 2 extra 1 liter bladders will weigh about 3 more ounces. Then you need an insulating container for the bottles. Dunno the weight, but I'd guess at least 8+ ounces to be of any use, especially in cold climates when you'd actually need extra warmth. You also need to heat the water. If you use a fire, then you're not really out any extra weight, but if you have to use fuel, then you're talking an ounce or so per night. If you have to melt snow, then there's more fuel used. Then there's the hassle. Time and energy is used by you to collect, heat, and store said water.
So to do all of this, we're talking at least another pound of weight (depending on fuel and trip length), plus the need to do so. Would that weight not be better served in your insulating pieces such as your sleeping bag or pad?
I don't mean to disparage your thought, but I do question the practicality of it. If there is proof to the contrary, I will happily retract my doubts!