if you have 5 alts at lvl 60 level them all at the same time to 80.
this is WAAAY faster than boosting 60-80 two times.

lvl 60-70 is still quite fast, just quest grind your ass off.
use a quest addon (carbonite is my favorite) and just blast trough all of them
when you hit 68 head to northrend and continue the quest grinding.

if you want you can queue AV inbetween quests after lvl 60 if you like to get some pvp experience while still getting xp.