I'm afraid you've missed the point. DW pages are not displayed in monotype and as Michaelsy says multiple spaces are removed. Monotype is however used for editing and source code is stored in it. Use double spacing for editing (particularly if using vi) and let DW transform your text into whatever.
Here’s why: Back when we used typewriters, every character was given the exact same amount of space on the page. That meant the letter i was given the same amount of space as the letter m, even though it clearly didn’t need it. This is called monospaced typesetting and it’s, well, spacey. We needed that extra space between sentences to make it easier to see the beginning of new sentences.[My emphasis]
Word processors and computers and everything that is not a very old typewriter use mostly proportionally spaced fonts, which adjust spacing to the size of the letter. That’s why a proportional font can squeeze 12 letters into the same space where a monospace font can only fit nine:
From your second reference. Exactly what I've said, two spaces in monotype and let the word processing sort out the finished document.