I guess one could model it after the old DnD (Man, I hate having to reference that game all the time, but it was the leader in this industry....) psionic combat. Where the majority of the attacks and defenses were only usable agianst other psionic characters. I like the idea that two magicians can be having a totally personal battle with each other while a different battle rages around them.
I would just suggest you make sure that it doesn't remove them from situation in the way the old netrunning rules did. It blows having to wait there while the decker (mage) deals with the complex grid (magic wards) or another decker (enemy mage). That is something that I really like about the current edition of Shadowrun, the netrunning and astral stuff can happen along side the normal stuff.