How can I raise a concrete bridge footing two inches?
I don't think I'd try to pour new masonry in such a thin quantity. It's likely to crack up due to moisture, vibration, etc. Hydraulic cement is more of a patchwork product, in my experience, and self-leveling cement is way off the mark. 

I'd do one of these:


Lay a paver brick (or a series of them) on the existing footing using landscape block adhesive, bolts and expansion anchors, or other retainers. This should be just large enough to support your bridge so as to not look gaudy. 
Build a wooden "leg" that coordinates with your bridge. It should be mounted flush on the outside as a deck post would, or slightly inboard, to minimize its appearance. 
Fabricate a metal leg. Box tubing and/or angle stock and a few bolts would do well. Either let this rust for a nice patina or protect it with enamel or paint-on galvanizer. 


To tell you which I'd select I'd need to see more of the situation.