[atul]

Originally posted by rabelais:
I'm thinking specifically of Banner of the Northern Star

(No cast message ... can I cast Light of the northern star for a double bump?)

And Crystal Shield (Can I cast Power of the spheres on top? etc.)

At least when I once tried to have my Crystal Shield -wielding astral mage to cast Power of the Spheres, he cast some other spell instead (Doom on enemy main army - I can live with that decision). So I'd guess items duplicate spell effects and aren't cumulative. Also, in the beginning of the combat you can see appropriate spell visual effects on item-wielders (banner, shield, SoS, etc) even if there aren't any Messages displayed.