This is rather simple but some people might not have thought about this.
To do this have the veteran account (the one who will be granted levels) on the realm you want, and then the recruited account on another realm. Use the real-id to become best friends then into the same group. Zone into any instance. If you do not see each other, you can use the guild summon (RAF summon does not seem to work) and you will both be together. You can now grant levels even though you are from different realms.
This might be useful if you leveled on a realm then changed your mind about it or find it easier to powerlevel on one realm then move to a fresh one.