From what I can see of Google Glass the single display is offset. Adding another display would give you two displays, each offset. That'd be awful for 3d vision, giving terrible headaches and other disturbances.

But, sure, it's probably possible to have two displays in front of your eyes that are mostly transparent when not in use and have readable information when being used. Pretty scary if you're doing anything though.

It's a non-trivial problem.