When a model is in more than one zone, does it count as being in both for purposes of dominating or only one?
You mean like for Quadrant Control? The model counts for one zone, whichever it is in the majority of.
ITS is fairly consistent in requiring a trooper to be more than half inside a zone in order to dominate it (not just touching), but if you're playing other mission types you might have other definitions of what's inside depending on the scenario. This may or may not result in a trooper being able to claim two zones.