You have got to be kidding me, right? Of all the things you could worry about in writing—characters, plot, theme and metatheme, moral center, rising and falling tension, use of language, etc. — you’re obsessed with this?
Once upon a time, when typesetters used single-letter type or operated linotype machines with “hot lead” type, two spaces made sense. Anything that made the typesetter’s job easier made sense. Editors were used to seeing those double spaces after a final period and a single space looked “wrong” and “sloppy” and – heavens! – amateurish, because it was not the norm.
Word processors have changed all that. It’s trivial to do a global search-and-replace two spaces for one. Your editor can, with a couple of clicks, make your manuscript look however she wants. (As an editor, I do this quite a lot and I don’t find it in the least annoying.)
What matters and what has always mattered is not however many spaces you put between sentences. It’s what’s in those sentences and how they fit together to create a story.
Here endeth Deborah’s rant on the subject of double spaces between sentences.