Currently evolutions already are an array, with zero or more tuples [type, parameter, evoSpecies]. For item split evolutions you could just add them there, but be triggered by different items.
For levels I'm thinking the easiest would be adding new evolution types, e.g. evotype_LEVEL_SPLIT_PHYS_ATK and evotype_LEVEL_SPLIT_DAYNIGHT_PHYS_DEF for a monster that becomes one species when its ATK is higher and another if its DEF is higher. These come in pairs (or triplets, etc), one type for each "branch" of evolution and then you'd give the monster one evolution tuple for each branch. (So e.g. [[evotype_LEVEL_SPLIT_PHYS_ATK, 10, mon_SWORDY], [evotype_LEVEL_SPLIT_DAYNIGHT_PHYS_DEF, 10, mon_SHIELDY]] as the evolutionlist)
The script amp_earn_exp is also responsible for keeping track of pending evolutions, and currently it only checks for evotype_LEVEL here. You would add the new types here as well, but have them only enqueue the new evolution if the condition for that "branch" is met (and then make sure to write conditions so that only one can be met at a time)