I never use two spaces between sentences.
Why? Because it’s wrong to do so.
Who says so? Well, typographers and professional publishers – the people whose job it is to print the written word.
Oh, and also look at any professional English writing style guide (e.g. the Chicago Manual of Style) because they’ll all say the same thing.
For more, read Farhad Manjoo’s recent article in Slate, ‘Space Invaders’.