Blame this.
Horatio’s Stereoscopic Infallible Viewer
This spell was designed by ‘Horatio,’ who was one of those interesting mages. Thoroughly interested in mind and body spells, it was… and ‘it’ is used deliberately; Horatio had a habit of switching bodies like some people switch shirts. Ethically, mind you: the mage was rather good at jump-starting a recently-deceased corpse and getting everything running again properly. Or possibly ‘is:’ nobody’s heard from it recently, but that could just be Horatio not bothering to tell anyone, right?
Moving along… you need three targets for this spell. Preferably, they should all be from the same species (high-intelligence ones are not preferred), and as closely related to each other as possible (kittens or puppies from the same litter are typically ideal). One target is designated as the ‘archer:’ the other two are ‘scouts.’ The caster takes up residence in the brain of the archer, and can cast spells at whatever both ‘spotters’ can see, with difficulty and range based off of the archer’s location. That’s one of the advantages of this spell.
The other? Well, it’s almost impossible to get any kind of mystical ‘lock’ on the actual caster this way, so a lot of counterspells and other defensive magic won’t work on spells cast this way. Essentially, you could still deflect a fireball cast using this method, but, say, fending off mind control attempts is a bit harder to successfully do, and trying to trace the spell back to its owner is going to be pointless; you’d just bounce from brain to brain to brain, and none of them will have the information that you need anyway. Or be able to comprehend it, probably.
As you might imagine, this is a fairly useful spell. It’s also insanely dangerous: simply kill one of the targets of it, and the caster is going to get immediately thrown out of the link – and probably have a stroke in the process. And the longer the caster stays in the linked minds, the harder it is for the caster to break the link consciously. And, of course, you need a serious amount of magical energy to even create the link in the first place. This is barely within the capabilities of a senior journeyman mage; and while easier for an archmage, one wouldn’t cast it frivolously. Even Horatio only used this spell once it got the hang of on-the-fly reactive healing spells…