The fastest leveling will be to RAF link the accounts.
As in, one existing account goes to account management, clicks the "invite a friend" link, and the second account uses the emailed cd-keys to start the game.

While the two accounts are grouped and within 5 levels, they receive 300% (triple) experience for quests and kills.
If you have access to a high level toon, not on either of these accounts, you can "boost" with the non-linked account for even faster leveling.

The RAF ends at 60th for a given set of toons, but in the 90 days the accounts are linked, you can level as many sets of toons as you'd like.

Also, for every 2 levels the new account gains, ie 3/5/7.../57/59... (28 pairs of levels), they can "grant" a level to a toon on the refering account.
So if you level a toon from 1st to 60th, they can give 28 levels to a toon on the parent account. Level two toons and they can give 58 levels.
The limit is, granted levels have to be given in the 90 day account link period and you cannot grant to a toon higher then yourself or to a toon at 60th.

If you were willing to RAF a new account, you could move an existing priest to the new account for 25 bucks.
The toon is not playable for an average of 4 hours.
After which point, the two priests can play together for triple exp's, until 60th.

Not sure this is worthwhile to you or not, but it is much faster leveling then without.
My RAF'd toons (with boosting) hit 70th in under 30 hours /played, and that's without any experience boosting or RAF leveling.
I even granted a toon 58 levels since then, just leveled the toon to 98% into level 2 and gave them 58 boosts, so they're not 98% into 60th, with under 5 minutes of played time.