The answer is only one: both must die together.
The "Heavenly Voice Obedience" must be used twice; this is the safest method. Of course, if the Hand Eye is telling the truth and Isaiah is lying, then the first "Heavenly Voice Obedience" will take Isaiah's life, and they can recover the four Corpse Blocks from him, eliminating the need for a second. If Isaiah is telling the truth and the Hand Eye is lying, then after killing Isaiah, using a second "Heavenly Voice Obedience" to deal with the Hand Eye will be enough.
Though this price is a bit hefty, it is indeed the most reliable.
In this regard, Diana and the Prison Director share the same view.
The difference is that the Prison Director gave Isaiah a chance for "self-preservation," while Diana wished for immediate execution, regardless of whether Isaiah is wronged or not, to eliminate any risk of suicide from the desperate man.
