You can only give lvls to characters that are under lvl 60. This means you can only boost chars to lvl 60 with the Bonuslvls. I think the fastest way is to get a Warry and Pally and boost them through instances with a lvl 80 char until they hit lvl 60. When you are just dualboxing, just go and quest your way up to lvl 70. If you boost the chars through instances you can hit lvl 60 within one and a half day of played time.