That is exactly what I do when i use the pre-made "popper" type packs. Just pour into the bottle and drop it into a container of hot water to warm it up. Put the rest into another bottle and pop it in the fridge. Use it within 24 hours and all good.

I think, and don't quote me on this and please if anyone knows this to be incorrect please correct me, but once you've heated ANY formula once, you cannot heat it a second time if it goes cold.