Really? I was under the assumption it was levels gained together. Though reviewing the FAQ shows that there's no mention of being forced to level together, that's just the 3x EXP boost.
Yes, it is possible, as others have said. I would like to add one thing. The recruit account's character and the veteran account's character does need to be on the same server.
However, you can benefit from the 3x bonus xp in cross-server groups. You just can't grant levels unless you're on the same server. How the levels were obtained by the recruit account (grouped or not) doesn't matter.