Step one don't level as holy. Shadow is your best bet for leveling.

What I have found trying to level my priest is that you can't do the same things you've done as a dps class. Where as my mage could routinely take on mobs three levels higher than him (and could have out damaged mobs even five levels over him if it wasn't for resists) a priest is lucky to take on equal level mobs. So as a priest you have to stick with greens. Unless it's undead mobs, and then you can probably go with yellows.

Best suggestion I can give though is to not do two priests together. I'd go with another healing class but one that can tank a little. Priest, paladin would probably be the best combo, but priest druid would probably be pretty nice too. I really couldn't tell you how a priest shammy combo would do.

Here's a guide for you. http://www.wowwiki.com/Priest_builds