Just throwing this concept out here

[Agrajag]

The whole metal/crystal/soul thingey is already in the game, in a way. Its those spells that have more than one path as a requierment.
I understand this isn't exactly what you want/mean, but I just don't see a point in just adding extra artifical paths when we can use what we already have.
I will say this though, there aren't enough dual-path spells. (imo)