This isn't really a plain language question—it's a question of style. What's the answer? It's true that years ago, everyone used two spaces after a period and before the beginning of the next sentence. But these days, the rule is that all types of written material should use one space. In fact, if you at look at Chapter 2 of the USGPO manual of style, it says:
A single justified word space will be used between sentences. This applies to all types of composition.
The FAA Plain Language Office uses these manuals for answers to questions about plain language, style and grammar:
Plain Language Questions
If you've got comments or questions about this, please contact:
Dr. Bruce V. Corsino
FAA Plain Language Program Manager
Phone: (202) 493-4074