You will lose the XP bonus on Acct 3 if you boost with Acct 1, and will lose the bonus on 5 if you boost with Acct 2.

Basically you need an UNLINKED account to boost to keep the RAF bonus intact throughout the group.

IE. 2 refers 3, 4 and 5.

1 boosts 2-5, 2-5 all get RAF bonuses, 2 gets the mount bonus only, however.

That's how I'm running and I've never missed an RAF bonus XP during any of my runs with either a 74 DK or 80 druid boosting.