Try to dual cast then stop dual casting, then dual cast again , do this very fast and you conserve mana and deal more damage (L1R1 + L1R1 + L1R1....)

Yeah, I already do that. I never invest into dual casting except for Destruction. When I don't need Impact, I simply cast like that.

It especially helps in the lower levels

Well yea. though its ridiculous that dual casting should be less efficient than casting it seperately; don't see this happening with dual wielding weapons do ya.