Why does dual-casting reanimate corpse turn zombie directly into ash
If you are dual-wielding (in the sense that you "charge" one of your hands, then "charge" the other hand, then deploy them both) then what's happening is: Your first spell is striking the corpse and begins to re-animate it while your second spell hits the zombie. Since you can only re-animate a corpse once, when the second spell hits the zombie it causes it to disintegrate (like it does when the time expires or the zombie is killed). If that is what is wrong, then you must "charge" both of your hands at the same time so that a single energy ball is created.