Here is my setup:

#1 (Vet) Lowbie
#2 (Link to #1) lowbie
#3 (Link to #1) 70 Mage I transferred for boosting.
#4 (Vet) lowbie
#5 (Link to #4) lowbie

In this setup I am getting:

#1 no bonus XP
#2 bonus XP
#3 No XP of course
#4 bonus XP
#5 bonus XP.

The moral of the story is make sure your 70 that is boosting is in no way related to the other accounts. I am cheating my #1 account out of bonus XP though #2 is getting RAF XP.