In 2nd edition AD&D, I've never been able to find any official statement on whether specialist wizards may cast spells that have both a permitted school and a forbidden school. While not directly stated, it's seems to be implied in The Complete Wizard's Handbook (WH) and The Complete Book of Necromancers (CBN) that specialist wizards may cast those spells.

For example, there are a few spells that belong to both Necromancy and either Enchantment or Illusion/Phantasm (the latter being prohibited schools for Necromancers in canon 2nd edition): Blackmantle (WH), Bone Club (WH), Corpse Visage (WH). Yet, in CBN, all of those are listed under "common spells for Necromancers" and some are on the list of memorized spells of the Necromancer NPCs in the book. This would seem to imply that as long as one of the schools is permissible, the spell should be available to wizard specialists.

As a DM, I would interpret the existence of spells with multiple schools are being more accessible to specialist wizards than less.

Does anyone know where this issue is addressed in any official capacity, such as a handbook or Dragon Magazine, etc.?