Autocorrect won't do what you want... you can put spaces in the With box, but not in the Replace box of Autocorrect. This is because one of the events that causes Autocorrect to fire is when a space is typed.

Maybe it would help you to tell Spelling and Grammer checker that 2 spaces should be flagged as wrong. To do so,
Tools > Options > Spelling and Grammer tab
Settings button
Spaces required between sentences dropdown, choose 1