[@Lawful Newtral] The only thing that they agree on is that Nerzar needs annihilated. 

Since neither of them actually created him, neither of them know how to destroy him (hence why they opted to simply imprison him). For the same reason, they are... [i]concerned[/i] with his reappearance, for they fear that he would be the only creature that could destroy them.

Not to sound Star Wars-y, but both gods would sense the disturbance and would honestly have to put their own bickering on hold to find and re-imprison him, even if it includes having to kill Cyndia.