First of all, I can't even imagine that happening in AZ unless the death penalty is abolished nationwide, in which case they'll probably make it retroactive. But to answer your question, I think lawmakers could set it up either way.
Let me say first that we have no reason to think any juror is going back on his/her word in voir dire. It is simply not true that any juror who is willing to vote for death would vote for death in this case. There is a lot worse evil than Jodi Arias on AZ death row, believe me. My first job in law school included reviewing and cataloging every holding on every issue in every AZ death penalty case, and the nightmares were awful.
In the guilt phase, if the jury can't agree, it does not automatically make the verdict "not guilty." It means there is a hung jury and the case can be retried to a new jury--just like in this phase. So there's nothing unusual about that part of the process. What's unusual is that, until a few years ago when the Supreme Court came out with the Ring case, these decisions were made by judges, not by juries. At least that's how it worked in AZ--I don't know if other states were using juries for the penalty phase.