Concerns of a new multi boxer

[Scurvi]

Secondly, I worry that by using this method to quickly level up my Hunter and Priest with the x3 bonus.. I will get to 60 and have a Priest that I've never really played. Right now the Priest is purely there to heal me and reap the XP benefits. Is this going to present me with a major problem at 60, or would I quickly learn the class 60-70 if I solo it? I'm intending on only multi boxing to 60.

What is your goal with your priest?
Most priests spend the majority of the time soloing to 60 so lacking group healing experience is common. You can do some research before you group but I found that practice was the main thing. The skills for group healing are: reaction to dmg, threat management, mana management, line of sight, healing priority. The last one was the hardest for me, all the research kinda went out the window when I was confronted with my first Mage-Who-Ends-Up-Tanking.

For soloing skills, I'd recommend experimenting with a few core spells while your dual boxing. You probably won't work them into your grinding routine but they're useful for "Oh No" moments.

Power Word Shield:
High mana cost, instant cast, can be used as a sort of no threat heal (applied out of combat before Mob is aware of the target). Will prevent spell pushback while its up.

Mind Blast:
High/Medium mana cost, short cast time, high threat. Can be used as a taunt (though its rare that you want to taunt with a priest)

Racial Skills:
Very situational, but its good to know what yours do.
http://www.wowwiki.com/Priest_Spells#Racial_Spells

Autowand:
Ignore the scoffs of mages/warlocks. This is useful, and a common part of priest grinding. Its worth noting that shadow dmg wands are boosted by Shadow Form and Shadow Weaving.

Levitate:
Survive large drops and jog across water. requires light feathers. Not really usable while duoing but I find its worth the loss of a bag slot while soloing (depending on the area).

Also you have the oppertunity to get used to the short heal, long heal, heal-over-time differences while duoing. Worth playing around with.

As others have said, going shadow spec is a good way to up the dmg of your duo. VE combined with SWPain and or mindflay provides a nice HOT that will heal the priest, hunter and pet at the same time. You're also likely to be leveling as shadow from 60-70. If you are just speccing into it to play around for a few levels please be aware that 40-50 are not a good indicator of the usual experience. They are however very very fun (a mob 5 levels above me? I can take him!)

Also I'd start soloing in EPL, Winterspring and/or Silithus rather than jump stright to outlands. The learning curve will be much kinder that way. (and EPL gives shackle practice).