You can make a dual-wielder, but I feel that for a shadowblade, dual-wielding is a bit less important than it is for a Rogue, since their role is more than straight-up damage, and dual-wield takes a fair amount of focus to get the AP cost down. Shadowblades also will generally have less Speed, and therefore less AP to spend.
If you have to drop a school, Air is better to drop than Witchcraft I think. Air has less great stuff for a Shadowblade and you can get away with just picking up a couple novice skills like Thunder Jump and Shocking Touch.
