Fast leveling advice needed

I want to level my warrior (acct1) and priest(acct2) as fast as possible to 70 they are 64 at the moment. I have a 70 lock on another account that I could boost with but what I am not sure about is if I should use the lock at all, if I should will it be better grouped or not?

Ultimate goal is to level all 3 through 70-80 together.

If your lock is 70 then leave it out of group.

Tag the mob with your warrior. Then Dot it up with the lock.

The mob will die quicker, you wont lose any XP either.