[Yvelina]

Ghost riders, Horrors and Phantasmal Attacks will stack, I did it yesterday. All annonymous spells will stack, and all non-annonymous spells will stack. I.e. if you send Angels, Calls of the Wind/Wild, Imprint Souls, and some teleporting mages, they will all attack together.

Funnily enough, when multiple players send annonymous spells at a province, the resulting hordes attack together, but only one player gets the battle replay.

As for the spells that target a commander, as opposed to a province, for example 'Visitation', 'Vengeance of the Dead', 'Earth Attack', etc... these do not stack in my experience. I do not know whether they even get targets assigned before the previous ones are resolved.