With the Jamba addon, you can open the quest window on both toons and have the slaves follow the master's choices.
If they're both equally geared same class, you can have the slave toons "mirror" the master for reward choices as well.

With RAF and no boosting, questing will be your fastest leveling progression.
But if that is boring, you could try instances for a bit.

With boosting, instances are probably your fastest leveling, provided the boosting toon is ahead of the challenge of the instance.
By that I mean they find the instance to be trivial for the level and gear.