Tall water bottles can't fit into a lot of washroom sinks. I have 820 mL bottles, and have a lot of trouble. The way I do it is to fit them in sort of sideways, so they can both be filled up to almost half way, then I use one bottle to fill the other one up to the top. Then I take the cap of the first bottle and use that as a little cup to fill up the second bottle to the top.