The GM that told you that a 70 doing the killing was not very nice, infact, down right lying. You can still boost with a 70 in the group that isnt linked to you, was doing it in Lbrs last night getting 700-950xp a kill at lvl ~45, rackin up 175k xp an hour.

The only time i have ran into not getting full bonus xp on some toons if if i do the turn ins, and then not thinking about it have them hop a FP or get too far away and then do the turn ins on the other toons. I am not sure why you're getting it on one and not the other at the same time though \