User blog comment:Hamengeri/Hamengeri's small item overhaul/@comment-24464243-20140915224124

I am not quite sure if I understand how giving Rabadon's Deathcap's passive a name allows buffing the active of an entirely different item.

I am also not quite sure if I understand how, should you be intending it, a unique passive and a unique active that share a name should interact.

Keep in mind that unique passives can stack should the effects that stack be entirely different abilities. For example, while Trinity Force's damage on next attack and Iceborn Gauntlet's damage on next attack + slow field on next attack are both under Spellblade, the Trinity Force will only override the damage on next attack part, and the slow field will still be created. Following that logic, you would still get both Deathcap and Deathfire's abilities, as increasing your ability power and nuking + increasing magic damage on a target are completely different. They may typically have the same function - increasing the damage you deal, namely - but they both have different nuances, such as Rabadon's affecting area of effect and non-damage effects while Deathfire Grasp only affects magic damage on a single target - although it can work on allied magic damage, too.

So perhaps a different method should be taken for what you're going for.