In which order R1, R2 and Akito the Exiled need to be watched?
Story of R1 and R2 isn't affected by Akito the Exiled enough to be considered a spoiler of any sort. 

AtE is an Alternate/Side-story and thus you can watch it whenever you want, though I'd advise to watch in the release order (and not just for Code-Geass, but almost every series out there) because that's how they are made and that's how you're supposed to be watching it.