Let's say a wizard has caster level 12.
If the Wizard casts both Consumptive Field and Greater Consumptive Field on himself, is his caster level 18 or 24?
The wizard will be level 18. This is assuming they can kill 6 creatures with the spells that they need to increase their evective level by 1 per death. Additional killed creatures will not do them any good.
If they kill less than 6 with the first of the two spells, they can kill additional creatures with the second spell and benefit, up to a total of 6 creatures from both. Details follow.
Consumptive Field says:
Additionally, your effective caster level increases by one per death caused by this spell, to a maximum increase of half your original caster level, improving spell effects that are dependent on caster level. (This increase in effective caster level does not grant you access to more spells, and it does not increase the duration of this spell.)
This spell functions like consumptive field, except that the field affects all creatures in the area with 9 hit points or fewer.
Both spells have the exact effect on your effective caster level, capped to a maximum increase of half your original caster level. And your original caster level is always the same. Original is defined in the dictionary as
present or existing from the beginning; first or earliest.
An enhanced, temporary caster level that you enjoy due to casting one of these spells, is an effective caster level, but not your original caster level. It is not the first or earliest version of your caster level.
Also, the spells do not increase your effective level by at most half your original caster level. The increase it to a level that has a maximum increase of half your original level. So they define not the increase, but the limit.
Therefore, the gain will never grow beyond that.
As your wizard is level 12, the maximum increase they can get is half of that or 6 levels. That will bring them to level 18.
I think this is not so much a question of stacking or not. Unless you fail to kill all six with the first spell, it does not matter if these count as bonuses, if untyped bonuses to your caster level stack or not, or if these count as the same source. Because either way, you run into the same upper limit.
In case a the first spell fails to get you to the full increase, because these are untyped bonuses, if they come from the two different sources -- the Consumptive Field and Greater Consumptive Field spell, additional deaths of the second spell would stack, until you reach the cap of half your original level.
Further castings of either spell would be from the same source you already used, and would not stack.