Generally speaking, a character can be himself up to the point where he starts using the name and power of his god. At that point, he has to think as the god rather than as himself. Of course, if a character is a priest of a god, he's probably already at least somewhat like the god, has that god's best interests at heart, and is going to remember that he's acting as that god's representative all the time, so he's kinda always on the clock, so to speak.
Kithairienites and Voraxians can potentially be friends, but when it comes down to calling on the power of the god, the Kithairienites are going to have to keep their god's blessings away from the Voraxians in most cases. If a Kithairienite is going to raise a Voraxian, she better have a really good reason for using her god's power to raise her god's enemy. That Voraxian better be absolutely required for the furthering of Kithairien's purposes (not the character's purposes) on Layonara. If she loses Kith's blessing for doing so, she shouldn't be surprised.
Short answer: Since Kithairienite Clerics are the enemies of Vorax, they probably shouldn't cast spells on Voraxians. Otherwise, if traveling with Voraxians for a while furthers Kithairien's purposes, have at. Just don't let them rub off on you.
