Smooth its in a lot of posts that the high level account must be unlinked:

EXP-Question: How can i max out my powerleveling in the remaining RAF-Time?


Also unless you hit some cap of some kind a higher level booster will take more exp, but not much I dont figure as 1000 divided by 80 isnt a lot less then 1000 divded by 70, but there might be a cap if you are using a high level to kill low level mobs. Instance is different then "real world (lol)" and is "not working as intended".

http://www.wowwiki.com/Formulas:Mob_XP